AstroPhysics Seminar

At the meeting point of galaxies with the large scale structure of the Universe lies the circumgalactic medium (CGM), the galactic diffuse atmosphere. Recent observations reveal that the CGM is ubiquitous, detected around star forming and quiescent galaxies, and that it is an extended and multiphase structure. However, due to the challenging nature of the measurements, many questions remain open - how much gas is out there, what are its thermal properties, spatial distribution, and morphology? These are linked to the properties of gas accretion onto galaxies, star formation, and galactic feedback processes, and are crucial to our understanding of galaxy evolution and how they affect the distribution of matter on large scales. 

In this talk I will describe the models I developed to address these questions. I will show what we learned from applying our models to observations of Milky-Way-mass galaxies, constraining the CGM mass, gas morphology, and energetics. Finally, I will present predictions for upcoming observations and future facilities (including CMB-S4, ELT, HWO, Athena, and more), which will allow us to test and improve our understanding of the physical processes governing the CGM.

Yakov is a candidate for a position in the Department.

There are ample ambiguities in the theoretical modeling of accretion flow around Sgr A*.
The theory and observations can not confirm the nature of the accretion flow around the central supermassive black hole.
In this talk, we investigate the possible application of low-angular momentum flow for the same.
We focus on the role of angular momentum in determining the properties of accretion flows around Kerr black holes.
Utilizing numerical simulations employing general relativistic magnetohydrodynamics (GRMHD),
We explore how different angular momentum profiles influence the flow dynamics.
Finally, we propose that intermediate angular momentum flows offer some insights into the complexities
observed in the supermassive black hole Sgr A*, which requires more study.
We also discuss other accretion possibilities around Sgr A* and compare them together.

The centers of many galaxies host supermassive black holes (SMBHs) surrounded by dense clusters of stars. To gain insights into the origins and growth histories of SMBHs, which often remain unclear, we need to study their environments. In this talk, I will present two recent projects that I have worked on. First, I will discuss a method for testing the “no-hair” theorem at the center of the Milky Way by measuring the spin and the quadrupole moment of Sgr A∗ using the orbital precession of S-stars. Unlike the original method, we show how it is possible to test the “no-hair” theorem using observations from a single star, and how to eliminate individual noise sources that may impede future quadrupole measurements.
In the second part, I will talk about the late-time evolution of magnetically dominated tidal disruption events (TDEs). In classic time-dependent, 1D accretion disk models, the inner regions—dominated by radiation pressure—are subject to viscosity instabilities. However, late-time observations of TDEs do not exhibit evidence of these instabilities. While the common theoretical approach to stabilize the disk relies on ad hoc modifications to the viscosity parameterizations, in this study we investigate a 1D alpha-disk model in which the pressure is dominated by magnetic fields rather than photons. In this talk, I will compare the models, focusing on the observable late-time optical, UV, and X-ray emissions.

I will review the Large Array Survey Telescope (LAST), a 48-telescope array based on off-the-shelf components, its classification and follow-up (unidentical) twin - the Multi-Aperture Spectroscopic Telescope (MAST), and the ULTRASAT space-borne UV telescope,  to be launched in Q1 2028. We will review the design and merits of each system, and focus on unique capabilities for specific science cases such as the detection of SNe shock breakout, and the search of planetary systems, pollution, and RV measurements of White Dwarfs.

Postmerger gravitational wave echoes provide a unique opportunity to probe the near-horizon structure of astrophysical black holes, which may be modified due to nonperturbative quantum gravity phenomena. However, since the waveform is subject to large theoretical uncertainties, it is necessary to develop search methods that are less reliant on specific models for detecting echoes from observational data. A promising strategy is to identify the characteristic quasinormal modes (QNMs) associated with echoes, in frequency space, which complements existing searches of quasiperiodic pulses in time. In this talk, I'll introduce the Bayesian search algorithm we've recently developed to detect these long-lived QNMs using phase-marginalized likelihood functions. The algorithm has been validated with signal injections in Gaussian noise for complementary echo waveforms. To work with real data, we've also accounted for the effects of non-Gaussian artifacts. We've conducted some of the proposed searches on real data from the first observing run of Advanced LIGO.

Note this seminar will take place only on Zoom; 

Active galactic nuclei (AGN) at high redshifts, observed when the Universe was only a few hundred Myr old, are crucial for our understanding of black hole growth in the early Universe and their co-evolution with galaxies. In particular, if gravitationally lensed and multiply imaged by an intervening strong lensing galaxy cluster, distant AGN can yield precious insight into AGN properties. We can even use them to constrain cosmological parameters by studying the time delay between the multiple images through the varying AGN activity. The advent of the JWST and its phenomenal near-infrared sensitivity and spatial resolution has initiated a new era in observations of strong lensing galaxy clusters and the lensed background sources. JWST observations allow us to push the frontier of observability towards fainter magnitudes and higher redshifts -- and thus uncovered a new population of dust-obscured red AGN at high redshifts.

In my talk I will present these red point-source objects observed in strong lensing fields with JWST (and archival HST data) that represent new and hitherto unobserved populations of AGN at high redshift. For one case in particular, at z=7 and which is multiply-imaged by the massive galaxy cluster Abell 2744, we obtained the deepest JWST/NIRSpec spectrum taken of a single object to date and detected its broad emission lines. This object opens fascinating prospects for future studies of its black hole properties through reverberation-mapping and even cosmological parameters through strong lensing cosmography.

In June 2023, Pulsar Timing Array (PTA) collaborations announced the groundbreaking discovery of a Stochastic Gravitational Wave (GW) Background at nano-Hertz frequencies. The origin of this signal remains uncertain, potentially arising from supermassive black hole binaries or from exotic phenomena in the early universe, such as first-order phase transitions, domain walls, or curvature peaks. Intriguingly, recent studies suggest that if the PTA GW signal has a primordial origin, then a non-negligible fraction of dark matter could be composed of solar-mass primordial black holes. These primordial black holes are currently under investigation by the LIGO-VIRGO-KAGRA interferometers. This highlights a fascinating connection between the nano-Hertz and kilo-Hertz GW observatories, both of which are probing the same class of early universe scenarios: violent exotic phenomena occurring around the epoch of quark confinement.

A gamma-ray burst (GRB) afterglow is considered an orphan when it is detected without a targeted search triggered by the prompt GRB emission. This can occur when the GRB jet points away from us or if the prompt emission along our line of sight is dim (e.g. a ``dirty fireball''). We present a semi-analytic model for the afterglow lightcurves based on and calibrated with numerical simulations. Such an approach better captures the peak time and flux for off-axis observers, as well as the shape of the lightcurves near the peak at different frequencies, which are the most relevant properties for transient surveys or targeted searches triggered by gravitational-wave detections. We use this model to calculate the rates of both single-epoch and multiple-epoch detections of orphan afterglows for different surveys as a function of their wavelength, sensitivity, and cadence. Additionally, we discuss an optimization scheme for observational strategies and potential methods for distinguishing orphan afterglows from other transients.

The linearized theory of gravity - the lowest-order terms in the Einstein Field Equation (EFE) - describes a force exchanged by a massless, spin-2 field, representing the perturbation of the metric relative to a flat background. The remaining, nonlinear terms of the EFE may then be thought of as self-interactions of that field. In this talk, I will present two advances that shed light on different aspects of gravitational theory, as seen from this “perturbative General Relativity” point of view.

I will first present a new way of deriving the EFE itself, taking the linear part as a starting point. As Feynman made famous, the self-interaction terms are expected even from purely field-theoretic reasoning, as follows. For consistency, gravity must couple to all forms of energy, including that of the gravitational field itself. So a gravitational energy term, quadratic in the field, must appear together with the other sources in the EFE. But the presence of this term then implies a cubic contribution to the field energy, and so on order by order, until we “bootstrap” to the full EFE. However, the idea of “the energy of the field itself” is not specific enough, on its own, to determine the exact terms. The derivations until now have proceeded in ways heavily inspired by our knowledge of Einstein’s theory and its symmetries, weakening the claim of a derivation from pure field theory. We show for the first time that no such knowledge is needed. We can get all of the self-interactions using only the requirement that our field equation should come from some Lagrangian. 

In the second part of the talk, I will discuss the quantum version of the theory. Perturbative GR can be straightforwardly quantized as a Low-Energy Effective Field Theory, with the spin-2 field interpreted as a “graviton”. We can then calculate quantum corrections to the classical theory of General Relativity. We have upgraded a program automating effective field theory calculations, called MatchMakerEFT, to enable working with spin-2 fields. Using this tool, we automatically compute some of these quantum corrections. We have also computed such corrections for an alternative theory called dRGT Massive Gravity, for which such corrections were not yet known.

(NOTE: Seminar is on Zoom only)

The formation of jets in black hole accretion systems is a long-standing problem. It has been proposed that a jet can be formed by extracting the rotation energy of the black hole (e.g., the BZ model) or the accretion flow (e.g., the BP model). Some important questions have not been answered: among these dynamical models, which one is correct? Whether these purely dynamical model can explain the observations of jet including the jet morphology? To answer these questions, we have performed general relativistic magnetohydrodynamic simulations of black hole accretion. Based on the simulation data, we have calculated electrons acceleration by magnetic reconnection in the jet. The predicted image of the jet is obtained by radiative transfer calculations and the results are compared to the morphology observations of the jet in M87. We find that the BZ-jet originating from a magnetically arrested disk around a highly spinning black hole can well reproduce the jet morphology, including its width and limb-brightening feature, while BP model does not work. The mechanism of magnetic reconnection in the jet is analyzed and found to be magnetic eruption in the accretion flow.

(NOTE UNUSUAL DAY AND LOCATION)

Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. The launching sites of these jets are surrounded by dense media, which the jets must cross before they can accelerate and release the high energy emission. Interaction with the medium leads to the formation of a mildly relativistic sheath around the jet resulting in an angular structures in the jet's asymptotic Lorentz factor and energy per solid angle, which modifies the afterglow emission.
We build a semi-analytical tool to analyze the afterglow light curve and polarization signatures of jets observed from a wide range of viewing angles, and focus on ones with slowly declining energy profiles known as shallow jets. We find overall lower polarization compared to the classical top-hat jet model. We provide an analytical expression for the peak polarization degree as a function of the energy profile power-law index, magnetic field configuration and viewing angle, and show that it occurs near the light curve break time for all viewers. When applying our tool to GRB 221009A, suspected to originate from a shallow jet, we find that the suggested jet structures for this event agree with the upper limits placed on the afterglow polarization in the optical and X-ray bands. We also find that at early times the polarization levels may be significantly higher, allowing for a potential distinction between different jet structure models and possibly constraining the magnetization in both forward and reverse shocks at that stage.

(Note: This talk is only on zoom)

I will discuss probing bosons of different spins, ie. scalar, vector, tensor particles (even if they are subdominant part of dark matter), specifically in the range e-27 to e-23 eV via pulsar timing arrays, e-21 to e-17eV via Supermassive black hole superradiance and e-12 to e21 eV via primordial black hole superradiance. If time permits, I can also discuss pulsar timing arrays and the astrophysics and new physics we can probe with them.

(NOTE UNUSUAL DAY AND LOCATION)

Understanding the east-west flows on Jupiter and Saturn has long been a challenge in geophysical fluid dynamics. Jupiter's atmosphere features distinct dynamical regimes: equatorial eastward flows, midlatitude eddy-driven jet streams, and a jet-free turbulent polar region. Saturn shows similar regimes, but lacks evidence for deep midlatitude meridional cells. While both planets have strong equatorial flows, Jupiter's reach ~100 m/s, and Saturn's are three times stronger and twice as wide.

Recent Juno and Cassini gravity measurements revealed that Saturn's zonal jets reach ~9,000 km deep, whereas Jupiter's are ~3,000 km deep. This study presents a new 3D deep anelastic model results, simulating the equatorial and midlatitude jet patterns, consistent with Juno's data. We find that vertical and meridional eddy momentum fluxes are crucial in driving midlatitude circulation. We also explore the parameters affecting the number, extent, strength, and location of midlatitude jets and cells.

At the equator, atmospheric depth is shown to control the strength and latitudinal extent of zonal flows, aligning with observations from Jupiter and Saturn. Our results suggest that atmospheric depth is proportional to the convectively-driven eddy momentum flux, influencing zonal flow strength. These findings offer a mechanistic explanation for the differences between Jupiter's and Saturn's equatorial regions, advancing our understanding of gas giant atmospheres.

Scalar fields are widely used in analyzing the cosmological phenomena such as inflation or dark energy. Despite its simplicity the use of scalar fields runs into various theoretical and phenomenological problems. I will demonstrate that going beyond the scalar field paradigm  opens a new direction of model building in cosmology. We consider the thermodynamical behavior of Banks-Zaks theory close to the conformal point in cosmological setting. Due to the anomalous dimension, the energy density and pressure of theory deviate from that of radiation and results various interesting cosmological solutions. The cosmological solutions from unparticles are the result of a collective emergent phenomenon not due modification of gravity or use of scalars. I will address the issue of initial singularity, fine-tuning of cosmological constant and cosmic coincidence problem. Finally, i will also discuss the resolution of cosmic tensions using the unparticles dark energy.

The talk will be based on these works

1.  https://arxiv.org/abs/1912.10532

2. https://arxiv.org/abs/2010.02998

3. https://arxiv.org/abs/2111.09946

4. https://arxiv.org/abs/2302.00067

Internal shocks are one of the leading dissipation mechanisms for powering the prompt phase of Gamma-ray bursts (GRBs). The basic paradigm is that a central engine produces a variable outflow wherein the faster trailing material collides with the slower leading material at a distance from the engine. Each such collision produces a pair of shock fronts, viz., a reverse and a forward shock front propagating in the faster and the slower material, respectively. In general, the physical conditions in both the shocked regions are very different. In my talk, I will show how, starting from a few basic central engine parameters, a self-consistent hydrodynamic solution of both shocked regions provides ingredients for understanding the spectra and the light pulses of prompt-GRBs. I will also show how internal shocks in structured jets can possibly provide a unifying theme for classical GRBs (CGRBs), X-ray riches (XRRs) and  X-ray flashes (XRFs).

TBD

Outflows in black hole accreting systems are not yet fully understood.  Some of the open questions are what is the driving mechanism, where is the wind located, what are the abundances in the wind. These outflows are observed near black holes, from supermassive ones at the center of galaxies to galactic stellar mass X-ray binaries. Although these black holes differ by orders of magnitude in scale, there are similarities regarding the physical process of mass accretion and outflows. In this talk, I will report on Chandra/HETG grating observations over the past two decades characterizing similarities and differences between these systems. Starting with a sample of outflows in nine active galaxies (AGNs) for which we find a universal dependence of outflow column density, NH, on the ionization parameter, xi. I will demonstrate the importance of the XRISM/Resolve spectrometer in constraining the column density of the high-xi components better than HETG, and with much shorter observations. A sample of X-ray binary outflows is built in order to search for common behaviors and compare with the AGN sample. First, I will present the results of the richest outflow observed in an X-ray binary to date. When applying the same analysis to the outflow of the X-ray binary GRO J1655-40 we find a complex ionization structure of the wind, different in nature from that of AGNs. We use this structure to measure abundances of eighteen different elements in the wind, leading to inferences about the historical supernova explosion. The outflow is characterized by peaks in abundances of odd-Z elements, which have never before been observed in the X-ray, and originate in a massive type II supernova with supersolar metallicity. 

Binary interactions certainly happen to stars. Interacting stars can lose or gain mass, forever altering their evolutionary histories. It is less evident that these processes leave a footprint in almost all fields of astrophysics. In this talk, I will give a few examples of such footprints. Red giants that have lost mass due to their companions can leave behind small helium-burning stars - their former cores - which appear as bright UV sources in the sky. The orbits of such systems clearly show imprints from the Galactic chemical history. In a closely related population, the cores of red giants stripped by a binary companion may end up in the instability strip, producing RR Lyrae stars relevant to the variable stellar community and cosmologists alike. Double white dwarfs, too, before merging, have a short interaction phase where the larger white dwarf loses mass. As we recently showed, this lost mass ends up in the well-observed early lightcurves of supernovae produced by double white dwarf mergers, informing us about their progenitors. I will conclude with a bold claim - any field of astronomy contains binary interaction signatures.

According to the jittering jets explosion mechanism (JJEM), intermittent accretion disks with varying directions launch jittering jets that explode core-collapse supernovae (CCSNe).  

Convective motion in the pre-collapse outer stellar core supplies seed perturbations to instabilities around the newly formed neutron star (NS), like the spiral modes of the standing accretion shock instability (spiral SASI), that amplify angular momentum fluctuations. 

These lead to accretion episodes via accretion disks onto the NS that launch several and up to few tens of pairs of jets along different directions, i.e., jittering jets.
In my talk, I will present a qualitative pipeline for analysis of SNRs demonstrated on G321.3-3.9, a SNR exhibiting a complicated structure, including point symmetries and "ears" in several wavelengths. 

I will detail the processing done and showcase the steps taken to examine the morphology of an SNR and denote the different features that could indicate the explosion mechanism, which we claim is the JJEM of CCSNe.

Understanding astrophysical phenomena like shock waves in neutron stars requires advanced physics theories. We investigate shock jump conditions in curved space-time, crucial for scenarios like neutron star combustion. Our analysis shows that general relativistic treatment is necessary, revealing that neutron star combustion always results in detonation. Additionally, we explore Diffusive Shock Acceleration, a mechanism behind cosmic ray spectra, using Monte Carlo simulations. We find that softer equations of state lead to wider spectral index ranges. Lastly, we utilize universal relations between frequency of oscillations, masses, and radii to unveil the core of neutron stars. These findings shed light on the underlying physics governing neutron stars and advance our understanding of extreme astrophysical environments.

Note: the talk will be given over Zoom, https://biu-ac-il.zoom.us/j/9290951953

Note also it is on THURSDAY

Wandering too close to a supermassive black hole is a dangerous path for a star, as it can get tidally disrupted. The growth of our observed TDE sample revealed that observed TDE rates are lower than theoretical rates, E+A galaxies are overrepresented among TDE hosts and a very low fraction of TDEs launch jets. 

I will first present our revised loss cone theory that can reconcile theoretical rates with observed rates. Before our work, TDE rate calculations considered only two-body weak interactions between stars. However, most stars undergoing TDEs come from within the MBH radius of influence, the densest environments in the Universe. Hence, we studied the impact on rates of close encounters: strong scattering, tidal captures and direct collisions. We found that with our revised loss cone theory and steep slopes of the stellar mass black holes, theoretical TDE rates are consistent with observed TDE rates. Additionally, our revised loss cone theory challenges popular explanations for the over-representation of E+A galaxies for TDEs.

While only a tiny fraction (~1%) of TDEs generate powerful relativistic jets, an increasing number of TDEs have been associated with delayed radio emission. I will next present our unifying theory for jetted TDEs where the misalignment angle between the spin axis and the star orbital plane can explain both the very low volumetric jetted TDE rate and the wealth of observations: highly relativistic and delayed radio emission.

ew data from different kinds of surveys will push our knowledge in all branches of cosmology and astrophysics. Even now, recent JWST data raise a curious puzzle: the large number of candidate-galaxies observed at high redshift suggest possible inaccuracies either in our description of the star formation history or in the cosmological ΛCDM model, that still have to be understood. 

Line intensity mapping (LIM) is poised to become a very promising tool in this framework. The synergy between different lines is capable of probing galaxies and the intergalactic medium across cosmic history. Moreover, LIM collects emissions from both bright and faint sources, hence being sensitive to structure evolution on different scales. 

In this talk, I will present some examples to show how LIM can shed light on our unresolved questions, starting from the JWST puzzle. On one side, I will discuss the detectability of signatures of different star formation models in the 21-cm signal of upcoming surveys. On the other, I will show how the voxel intensity distribution of future CO maps can be used to probe deviations from ΛCDM down to sub-Mpc scales, testing the existence of exotic cosmic components. If we are able to exploit its data in an optimal way, LIM will play a key role in the next step of our understanding of the Universe.

The Two State-Vector Formalism of Yakir Aharonov has yielded many surprising predictions about the dynamics of a particle during the time-interval between past and future measurements. However, these intermediate states, rigorously derived from standard quantum theory, are, by definition, impossible to validate by measurement. I present an experiment where this difficulty has been overcome, and a projective ("strong") measurement probed this dynamics. An intriguing interplay between positive- and negative-mass particles is revealed, shedding new light on the "bomb-testing experiment" and further into the nature of the wave function.

In the last decade, gravitational waves and multi-messenger time-domain astronomy provides a fresh view of the dynamic Universe and precursor a new era in astrophysics. Notably, it sheds light on the astrophysics of compact objects, the origin of the heaviest elements, and allows for unique probes of fundamental physics. Those heavy elements produced via the rapid neutron capture process have remained a question of intense debate for many years. A fresh example event is the "kilonova" emission that accompanied GW170817 revealed a binary neutron star merger. I will discuss recent results on the binary neutron stars simulation and how other explosive transient like the collapse of massive rotating stars "collapsars" which give rise to long GRBs and the formation of heavy elements in the universe. In particular, I will focus on two frontier research areas- neutron star mergers and collapsar (/massive- collapsar). Also, highlight how multi-messenger astronomy may answer how does the Universe create the heaviest elements?

In the past years, enormous progress has been made in the field of exoplanet transmission spectroscopy. New techniques and instruments like the JWST now enable the determination of the properties of planetary atmospheres, as well as the detection of molecular species. In order to search for life on other planets via the minute signatures of biomarkers, we need to make sure that as many potential sources of systematic bias as possible are accounted for in the transit model.

However, in all approaches to this day, the stellar "background source" is viewed as a homogeneous disk consisting of only a photosphere.

In my presentation, I will first show that this assumption introduces strong biases in the derived transmission spectra and then introduce TACHELES, the first transit model that incorporates both a stellar photosphere and a chromosphere.

The last decade has produced a number of remarkable discoveries, such as the first direct observation of gravitational waves by the LIGO/Virgo collaboration and the first black hole image taken by the Event Horizon Telescope. These discoveries mark the beginning of a new precision era in black hole physics, which is expected to develop further by future experiments such as LISA, the Einstein Telescope and Cosmic Explorer.

In the era of precision black hole measurements, there is a need for precision theoretical methods and accurate predictions. In this talk I will describe an integrable sector of the gravitational scattering problem - analogous to the hydrogen atom in quantum mechanics - in which exact predictions can be made, and the implications for astrophysical black holes and binary mergers.

In my talk, I’ll discuss how UV-optical observation of supernovae very early (<3d) in their evolution can inform us about their progenitor stars and the explosion itself. I’ll show how many calcium rich supernovae and stripped-envelope supernovae have a prominent early time behavior, probably related to the CSM they ejected weeks or months before the explosion, informing us about the explosion mechanism. I’ll also show how in Type II supernovae, the early UV-optical light curve can be used to map out the density profile of the progenitor prior to explosion. I’ll show this for the very well observed case of the M101 SN2023ixf, and discuss what can be learned from a sample of SNe II from the ZTF survey with early UV data. Using this sample, I will argue that while most SNe II have a shell of dense CSM affecting the UV-optical light curve, this is due to a luminosity bias, and up to 80% of red supergiant stars result in a supernova shock breakout from the stellar envelope itself. If time permits, I’ll outline our plans to observe such supernovae with the ULTRASAT survey, and what we can expect to learn.  

I perform a template-based search for stimulated emission of Hawking radiation (or Boltzmann echoes) by combining the gravitational wave data from 47 binary black hole merger events observed by the LIGO/Virgo collaboration. With a Bayesian inference approach, I found no statistically significant evidence for this signal in either of the 3 Gravitational Wave Transient Catalogs GWTC-1, GWTC-2 and GWTC-3. However, the data cannot yet conclusively rule out the presence of Boltzmann echoes either, with the Bayesian evidence ranging within 0.3-1.6 for most events, and a common (non-vanishing) echo amplitude for all mergers being disfavoured at only 2:5 odds. The only exception is GW190521, the most massive and confidently detected event ever observed, which shows a positive evidence of 9.2 for stimulated Hawking radiation. The ``look-elsewhere'' effect for this outlier event is assessed by applying two distinct methods to add simulated signals in real data, before and after the event, giving false (true) positive detection probabilities for higher Bayes factors of 1.5%, 4.4%  (35 ± 7 %, 35 ± 15 %). An optimal combination of posteriors yields an upper limit of  A < 0.4 (at 90\% confidence level) for a universal echo amplitude, whereas A ~1 was predicted in the canonical model. To ensure the robustness of the results, I have employed an additional method to combine the events hierarchically. This approach involves using a target gaussian distribution and extracting the parameters from multiple uncertain observations, which may be affected by selection biases. The next generation of gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic Explorer can draw a better conclusion on the quantum nature of black hole horizons.

The talk will begin 14:00 Israel time (13:00 Norway time). Note the unusual Zoom link: 

https://stavanger.zoom.us/j/67983114516?pwd=OUt6UGR6K1ZhdDl0MTBTUWpjckhldz09

 Meeting ID: 679 8311 4516

Password: 216067

The origin and mechanisms of Fast Radio Bursts remain enigmatic.
These events, mostly from the distant Universe but one at 3.6 Mpc and
another within our Galaxy, have typical durations of 1--10 ms but temporal
structure down to tens of ns and brightness temperatures $\sim 10^{36}$ K,
comparable to those of pulsars.  Some FRB sources have been observed to
repeat thousands of times but others appear not to repeat.  I will discuss
possible source models and how to test them.

Observations show that the energy density of our universe is dominated by dark energy, and was dominated by dark matter at the epoch of Matter domination. Therefore, it is quite surprising that the epoch of radiation domination was dominated by SM physics. Moreover, the constituents of dark radiation, namely light particles, are generic in QFT. Famous examples such as goldstone bosons, chiral fermions, and gauge bosons are common in many extensions of the SM. Therefore, from both the cosmology and particle physics points of view, one expects to see some form of dark radiation. Although we have experimental evidence that dark radiation cannot be the dominant component of radiation energy density, current data still permits the existence of a significant dark component. However, this situation is expected to change dramatically in the next two decades. Future CMB and Large Scale Structure experiments will have a sub-percent sensitivity to a component of dark radiation. Interestingly, current cosmological anomalies, especially the ~5𝜎 Hubble tension, might be our first hints of such an exotic type of new physics. In this talk, I will review the topic and present some previous and ongoing works about dark radiation.

Daniel is a candidate for the Department.

The talk will be given physically on campus, and also broadcast on Zoom meeting https://biu-ac-il.zoom.us/j/9290951953 (and recorded)

In this talk I will develop a method to constrain the Cosmological Constant Λ from binary galaxies, focusing on the Milky Way and Andromeda. I will provide an analytical solution to the two-body problem with Λ and show that the ratio between the Keplerian period and TΛ = 2π/(c √ Λ) ≈ 63.2 Gyr controls the importance of effects from the Cosmological Constant. The Andromeda-Milky Way orbit has a period of ∼ 20 Gyr and so Dark Energy has to be taken into account. Using the current best mass estimates of the Milky Way and Andromeda galaxies, I find the Cosmological Constant value based only on the Local Group dynamics to be lower then 5.44 times the value obtained by Planck. With future astrometric measurements, the bound on the Cosmological Constant can be reduced to (1.67 ± 0.79) ΛPL. The results offer the prospects of constraints on Λ over very different scales than previously. With other binary systems I show that the upper bound on the cosmological constant decreases when the orbital period of the system increases, emphasizing that Λ is a critical period in binary motion.   

based on Astrophys.J.Lett. 953 (2023) 1, L2

This talk will be given on Zoom: https://biu-ac-il.zoom.us/j/9290951953

Our very own Asaf Peer will give career advice for academic Astrophysics tracks

In the past few years, the Event Horizon Telescope has released the first close-up interferometric images of two supermassive black holes, M87* and SgrA*. It is believed that within these images is embedded a fine, yet-unresolved brightness enhancement called the photon ring. The ring is a universal consequence of strong lensing by the black hole and thereby conveys information on its spacetime geometry, potentially providing a new independent avenue for tests of general relativity in the strong-field regime. In the talk I will review the theory of the photon ring and its corresponding spacetime region, the photon shell, which governs the universal lensing structure. I will then describe some current efforts and future prospects for resolving the ring, which include both the construction of more powerful instruments and the development of novel analysis methods.

We perform population synthesis of massive binaries to study the mergers of neutron stars (NSs) and black holes (BHs) with the cores of their giant secondaries during common envelope evolution (CEE). We use different values of the efficiency parameter $\alpha_{\rm CE}$ in the framework of the energy formalism for traditional CEE ($\alpha_{\rm CE} \leq 1$) and including additional energy sources to unbind the envelope ($\alpha_{\rm CE} > 1$). We constrain the possible values of $\alpha_{\rm CE}$ by comparing the results of our simulations with local rate densities of binary compact object mergers as inferred from gravitational waves observations. We find two primary evolutionary pathways of binary systems that result in NS-core mergers, while only one of them can also lead to the merger of a BH with the core of the giant star. We explore the zero age main sequence (ZAMS) statistical properties of systems that result in NS/BH-core mergers and find that the two evolutionary channels correspond to a bimodal distribution of orbital separations. We estimate the percentage of the mergers' event rates relative to core collapse supernovae (CCSNe). We include the effect of mass accreted by the NS/BH during CEE in a separate set of simulations and find it does not affect the mergers' event rates.

In early 2016, Batygin and Brown announced the likely presence of a ninth, large planet in the outer Solar System, based on the properties of the population of Trans-Neptunian Objects. Since then, multiple teams around the world have tried to detect the predicted planet, including myself.

In my presentation, I will give a brief outline of the underlying theory, followed by a description of my approach and an update on the current status.

In this seminar I will introduce the Large Array Survey Telescope (LAST), provide an overview of its current status, outline our gravitational-wave follow-up plans during O4, and highlight other, ongoing scientific projects. LAST is currently under construction at the Weizmann Astrophysical Observatory (http://www.weizmann.ac.il/wao/) in southern Israel. It is a cost-effective and flexible telescope array consisting of 48 commercially available telescopes. Its modular design enables various observing modes: The co-aligned configuration corresponds to a 1.9m telescope with a 7.3 square degree field of view, and in the open mode it can cover 350 square degrees instantaneously. This will allow rapid and efficient follow-up even for poorly-localized gravitational wave triggers. Therefore, LAST data will help to find kilonovae earlier and add crucial early data points to the light curves.

LAST is the first of several future telescopes at the observatory: While LAST surveys the entire sky to search for transients, the PAST and MAST arrays will obtain photometric and spectroscopic observations for discovered sources. All telescopes will in addition provide ground support for the ULTRASAT mission.

The New Weizmann Astrophysical Observatory (WAO), located near Kibbutz Neot Smadar in the South of Israel, will host two telescope arrays: The Large Array Survey Telescope (LAST) and the Multi Aperture Spectroscopic Telescope (MAST). 

In the talk, I will provide an overview of the observatory, with a particular emphasis on HighSpec, a high-resolution spectrograph, and MAST, the telescope array it will be coupled to.

HighSpec offers a high spectral resolution of R ∼ 20, 000 while maintaining an exceptionally high throughput with a peak efficiency of ≳ 55% over a narrow band-pass of ∼ 100 − 170 A for selected spectral lines. This is made possible by using highly optimized ion-etched binary mask gratings and an electron-multiplying CCD (EMCCD) detector.

In addition, I will review the science cases for which HighSpec has been optimized. Its role as a follow-up instrument, supporting both LAST and ULTRASAT, will also be discussed. 

By adopting a bottom-up instrument design approach tailored to address specific astrophysical questions and leveraging the observing flexibility provided by the MAST infrastructure, allowing for both parallel and combined observations, HighSpec, along with its twin instrument DeepSpec, emerge as powerful and unique tools in the field.  

HighSpec is currently undergoing the fabrication phase,  with commissioning expected to commence by mid-late 2023.

General relativity predicts that the interior of black holes is an empty region surrounded by an event horizon. This perspective was challenged by the realization that black holes radiate quantum mechanically and evaporate, which led to the so-called ''information paradox''. The resolution of the paradox introduces horizon-scale corrections to the classical black hole picture. In this talk, I'll demonstrate how by performing long-duration observation in the post-merger data immediately following GW150914, we are able to impose significant constraints on these corrections and put sub-atomic bounds on the horizon's position.

Apparent horizons are routinely used in numerical relativity to describe black holes in simulations of dynamical systems. Advances in numerical methods allowed us to follow these objects into the interior of merging black holes, revealing how the two original horizons connect with the remnant horizon.
In this talk, I will present results on head-on mergers, showing that the evolution of apparent horizons is much more intricate than previously thought: In the interior of the newly formed common horizon, the original horizons are individually annihilated by unstable horizon-like structures. This completes our picture of how two black holes become one and provides the analog of the famous pair-of-pants diagram of the event horizon now for the apparent horizon.

Most known stellar black holes are members of close binaries, where accretion-fueled X-ray emission was able to trigger their detection. However, if the separation between the two components is sufficiently large, the accretion rate drops, leaving the black hole in quiescence. Presumably, most black holes in binaries reside in this dormant state, waiting to be discovered. A promising detection channel for these systems is the astrometric wobble induced on the luminous star by its dark companion. In preparation for the release of astrometric orbits of Gaia DR3, Shahaf, Mazeh, Faigler, and Holl (2019) proposed a triage technique to identify hierarchical triples and astrometric binaries with compact companions. Having the astrometric orbits of Gaia at hand, we applied this technique to identify a large sample of hierarchical triples, hundreds of stars with a white dwarf companion, and dozens of systems with neutron stars or black-hole candidates as their faint massive secondaries. In this talk, I will describe the triage classification scheme, present the population of compact objects found in Gaia DR3, and discuss some prospects for future study.

The LHC collides protons at a rate of 40 MhZ to provide a good chance of an interesting event occurring. The collision energy is also extremely high at 13.6 TeV as the original purpose is to probe high energy physics. 
 
We can't afford to save every collision though, so we employ a trigger system that has to very quickly determine which collision events are worth saving for further analysis. Traditionally, the trigger will do a rough reconstruction of the collision, and if it meets certain criteria, it will save all of the data in the detector which can be used for a more detailed reconstruction later on. 
 
The event rate that we can ultimately save to tape is limited by the readout bandwidth which is a product of the trigger rate and event size. We can't afford to increase the trigger rate, but we can decrease the event size which allows us to save orders of magnitude more data. We do this through a method we call a "trigger level analysis" where we only save the objects reconstructed by the trigger, and nothing else. This comes at the cost of decreased reconstruction precision which we have to cleverly work around, but it allows us to explore a phase space of very rare events. The analysis we are currently doing searches for very rare and relatively light dark matter candidates which would be impossible by doing a traditional analysis. 
 

These days jets are observed on very different scales: from few gravitational radii to a few kiloparsecs. The variety of scales and variability times tells us that there are few dissipation mechanisms that transform electromagnetic energy into the observed radiation. In my talk I will discuss two possible mechanisms: (1) the energy dissipation in current sheets in the vicinity of the black hole and (2) the current-driven kink instability as a source of energy dissipation on the scales of 10^5-10^7 gravitational radii from the central engine.

Smadar Bressler (1), Johnathan Ziv Bressler (2), Giora Shaviv (1)

We model radiative transfer through planetary atmospheres, imposing energy conservation. We solve the heat transfer and the radiative transfer equations in the two stream approximation. As a first order approximation, we use a 2 band model defined by different averages of the optical depth in the UV-visible range τ(vis), and that of the infrared range τ(fir). We show that greenhouse gases that absorb in both ranges, especially where the optical depth is ~1, reduce the greenhouse effect . 

Thus, we search for additional energy sources for global warming and climate change. 

We find a resemblance between the pace of change of the global average temperature anomaly (GATA) and that of the average drift of the north magnetic pole (NMPD), or its separation from the south magnetic pole. We estimate the energy involved in complete pole reversal by assuming a double pole model, and show that it is several orders of magnitude higher than that required for heating of the Earth's system by 1K. We propose that the heating process associated with pole drift, may be connected with friction in the night reconnection zones upon changing of the field geometry, and is evident in the release of non-equilibrium shortwave radiation, and high energy particles which travel towards the Earth and heats it. We find a possible correlation between STEREO data from 2007 (Panchencko et al 2009), and the pace of drift of the NMPD, GATA and auroral intensities and frequency for the same period. 

We also address the question of the average direction of drift of the North magnetic pole. We find that the average drift direction is relatively close to the 1908 Tunguska event coordinates, and ask whether the greatest impact of the 20th century could have initiated the drift process. We calculate the energies brought to the Earth by flying-by asteroids, and show that their energy may suffice to warm the Earth's surface either by direct interaction or by causing pole drift by impact and heating through magnetic energy released in reconnection events.  

1 Dept. of Physics, Israel  Institute  of  Technology, Haifa, Israel

2 Dept of Physics, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, Israel 

Accretion of matter onto compact objects is tightly connected with ejection of matter from them.  After all, it is the accretion flow that feeds the outflow.  Despite many efforts, this connection has not become clear.  In particular, the observed correlation between the radio and X-ray fluxes in the hard state of black-hole X-ray binaries (BHXRBs) has been around for more than two decades now. It is currently accepted that the hard X-rays in BHXRBs come from Comptonization in the corona and the radio emission from the jet. The jet and the corona, however, are separate entities with hardly any communication between them, apart from the fact that the jet is fed from the corona.  It is also widely accepted that the accretion flow around black holes in BHXRBs consists of an outer thin disk and an inner hot flow. From this hot inner flow, an outflow must emanate in the hard and hard-intermediate states of the source. By considering Compton up-scattering of soft disk photons in the outflow (i.e., in an outflowing "corona") as the mechanism that produces the hard X-ray spectrum, we have been able to explain quantitatively a number of observed correlations. Here, we investigate whether this outflowing "corona" can explain the observed radio - X-ray correlation also.  
We consider a parabolic outflow and compute the radio emission at 8.6 GHz coming from it, as well as the power-law photon-number spectral index Γ of the Comptonized hard X-rays produced in it. Thus, we have a correlation between the computed radio flux F_R at 8.6 GHz and the computed spectral index Γ of the hard X-ray spectrum. This correlation is actually a theoretical prediction, since both F_R and Γ are computed from the model and, to our knowledge, no such correlation has been constructed from observations for the hard and the hard-intermediate states. This prediction can be confirmed or rejected in future outbursts of GX 339-4. From observations of GX 339-4, we also produce a correlation between the observed X-ray flux F_X and the observed index Γ. Thus, for each value of Γ, observed/computed, we have the corresponding values of the observed F_X and the computed F_R, which we plot one against the other.  We find that, in the hard state of GX 339-4, our idea reproduces the observed correlation of F_R ∝ (F_X)^0.6 . In addition, in the hard-intermediate state of GX 339-4, we predict that, in future outbursts of the source, the F_R will exhibit first a sudden increase and then a sharp drop within a very narrow range of values of F_X . Also, since in a parabolic outflow the density is largest at its bottom, the transverse optical depth at the bottom of the outflow is very large (typically 10 to 100) and so the soft input photons, that are Comptonized there, see something like a "slab" above their emission. This may explain naturally the observed X-ray polarization from BHXRBs.

An inflow of interstellar neutral particles known as the interstellar wind is created as the Sun moves through the interstellar medium. This influences the composition of the Heliosphere, or the region of space affected by the Sun. After these neutrals enter the Heliosphere, they may become ionized by processes such as photoionization, electron impact ionization, and charge exchange with the solar wind. These “pickup” ions propagate radially outward with the solar wind. Studying distributions of these pickup ions is one of the most accessible means of studying Heliospheric ‒ interstellar interactions. One such feature that we measure to determine characteristics of this flow interaction is the helium focusing cone. Due to the high first ionization potential of helium, the focusing cone, a signature of enhanced helium which has been gravitationally focused on the downwind side of the Sun, leads to a measurable enhancement in the pickup ion population out to 1 AU. The Solar Orbiter Heavy Ion Sensor (SO-HIS) measures ions in the range H+ to Fe20+ with sufficient energy and angular ranges to measure characteristic signatures of pickup ions in the velocity distribution. To achieve our aim to study Heliospheric ‒ interstellar interactions using in situ pickup ion measurements, we present a characterization of the SO-HIS instrument required to enable identification of pickup ions in the data. We characterize the geometric factor and solid state energy detector efficiencies and develop validated pickup ion distribution measurements.

TBD

.The accuracy of gravitational-wave models of compact binaries has traditionally been addressed by the mismatch between the model and numerical-relativity simulations. This is a measure of the overall agreement between the two waveforms. However, the largest modelling error typically appears in the strong-field merger regime and may affect subdominant signal harmonics more strongly. These inaccuracies are often not well characterised by the mismatch. We explore the use of a complementary, physically motivated tool to investigate the accuracy of gravitational-wave harmonics in waveform models: the remnant's recoil, or kick velocity. Asymmetric binary mergers produce remnants with significant recoil, encoded by subtle imprints in the gravitational-wave signal. The kick estimate is highly sensitive to the intrinsic inaccuracies of the modelled gravitational-wave harmonics during the strongly relativistic merger regime. We investigate the accuracy of the higher harmonics in four state-of-the-art waveform models of binary black holes. In this talk, I will present the results of our study and discuss how numerical-relativity kick estimates could be used to calibrate waveform models further. 

According to MHD, a strong parallel shock is a few mean-free-paths thick, with a density jump of 4. The talk will comment on these 2 aspects for the case of a collisionless shock. First, I’ll review recent analytical works explaining how, as the plasma becomes collisionless, the front continuously switches from a few mean-free-path to a distance much shorter, in terms of the upstream plasma parameter. Second, I will also show how the density jump of a strong parallel collisionless shock can shrink to 2 instead of 4. This is due to the ability of a collisionless plasma to sustain a stable anisotropy in the presence of a magnetic field. PIC simulations will be presented confirming this conclusion.

As a part of the ongoing search for inspiraling black hole binaries in gravitational-waves data, we derive a scheme to obtain the optimal detection test-statistic (the evidence ratio) in a way that is efficient to compute both on the target signal and compute its exact properties using time-slides (time-shifting the detectors with respect to each other). This detection statistic, for the first time, includes the effects of both precession and high emission modes. Due to computational limitations, we employ the test for the 10^4 most promising candidates from the regular (fast) match-filtering pipeline. This is an improvement in both statistical significance and run-time compared to a previous scheme by the GW@IAS collaboration (Zackay et al. 2019). A key component of the new scheme is posterior probability estimation, commonly performed today by sampling algorithms (e.g. MCMC, Nested Samplers). Time permitting, we'll discuss recent developments in this area.

The origin of the Fermi/eROSITA bubbles still remains a debated question. One of the popular theories is that the bubbles are driven by a past AGN jet from the Sgr A*. A crucial assumption for the claimed jet is that the jet is launched along the rotation axis of the Galaxy despite several observational pieces of evidence indicating a different picture. In this talk, I will discuss the general effects of a misaligned jet in the Milky Way ISM. We find that the dissipation of the claimed jet within the ISM is an essential requirement for simultaneously producing the symmetrical features of the Fermi/eROSITA bubbles and the observed x-ray signatures. We show that hydrodynamic jets from Sgr A* fail to produce these features and therefore are ruled out. Other Blackhole driven mechanisms such as a magnetic jet, accretion wind, or TDEs, can in principle produce these features provided their power is ~ 10^{40.5-41} erg/s.

Sub-galactic astrophysical structure provides probes into the microscopic nature of the dark sector and its dark matter content. For example, dark matter with sizable self interactions and / or dissipation can leave distinctive signatures on the properties of satellite galaxies around Milky Way-like hosts. In a recent study, I placed novel constraints on a generic class of self-interacting dark matter models by analyzing a number of Milky Way dwarf galaxies. The results push these models into a parameter space with a very specific and new prediction: self-interactions within satellite galaxies can be either very large (so large that new dynamical effects become important), or very small (so small that such models are usually thought of as collisionless), but not intermediate. Specifically, if self-interactions are large, some dwarfs of the Milky Way must be undergoing a process of gravothermal collapse, and this process has a number of distinct observational predictions which can be searched for in current and upcoming data. The same models predict dissipation in certain regions of the parameter space; this offers additional observable signatures. In this talk, I will lay out a program to fully cover the parameter space of such models within the next few years by utilizing new theoretical understanding as well as upcoming observational data.

*Dr Slone is a candidate for a position in the department

We propose a novel way to challenge the boundaries of our knowledge of particle physics. The Standard Model of particle physics is a tremendously successful framework that is known without doubt to be incomplete. It fails to explain central phenomena such as neutrino masses, dark matter, and the baryon asymmetry of the universe. In order to tackle the question of what is the more fundamental Lagrangian describing the particle content of the universe, New Physics is being searched for in a variety of ways, from particle accelerators to astrophysical observatories. 

I will discuss recent progress in the "intensity frontier", and demonstrate how careful analysis of rare processes at low energies is potentially sensitive to physics at scales that far exceed the reach of present day particle colliders. I will review our proposal for a next-generation multi-purpose kaon experiment with the potential to (i) test less explored sectors of the SM, (ii) look for new particles and interactions, and (iii) study aspects of the strong interaction.

* Dr Dery is a candidate for a position in the department

*** Note unusual date and time ***

Importance of magnetic fields and gravitational effects, particularly the general relativistic one, is already understood to play key roles in many modern astrophysical processes including underlying gas dynamics. However, in certain features lying with the detection of black holes and white dwarfs, the combined effect of magnetic fields and general relativity appears to be indispensable. They are powerful jets in a black hole accretion disk, ultra-luminous X-ray sources in their hard states, super-Chandrasekhar limiting mass white dwarfs to explain extremely over-luminous type Ia supernovae significantly violating the Chandrasekhar-limit and Philips relation, massive neutron stars predicted by gravitational wave astronomy etc. I will attempt to uncover the key features of the combined magnetic and general relativistic effects, explaining the above stated enigmatic sources observationally.

This talk will be given remotely over Zoom, at https://zoom.us/j/9290951953

The time is 14:00 Israel Time, 17:30 India Standard Time

The number densities, structures, and internal dynamics of low-mass galaxies provide some of the most interesting clues to the nature of dark matter and the theory of galaxy formation on small scales. Up until recently, our understanding of low-mass galaxies has largely been informed by observations of dwarf galaxies that orbit our Milky Way galaxy. I will present novel observational efforts that now enable the discovery of such low surface brightness galaxies beyond our local galactic neighborhood. I will discuss some of the follow-up observations of these extragalactic low-mass galaxies, focusing on their dark matter content and intriguing globular cluster populations, revealing significant diversity and new astrophysical puzzles. I will conclude by discussing ongoing surveys that will be essential in mapping the census and properties of the general population of low-mass galaxies.

When two black holes merge, the late stage of gravitational wave emission is a superposition of exponentially damped sinusoids. According to the black hole no-hair theorem, this ringdown spectrum depends only on the mass and angular momentum of the final black hole. An observation of more than one ringdown mode can test this fundamental prediction of general relativity. Here we provide strong observational evidence for a multimode black hole ringdown spectrum using the gravitational wave event GW190521, with a Bayes factor of 56 preferring two fundamental modes over one. The dominant mode is the l=m=2 harmonic, and the sub-dominant mode corresponds to the l=m=3 harmonic. We present an extensive study of simulated signal injections that confidently supports the statistical evidence. Two methods are employed to search for quasi-normal modes in the data, using signal models that are agnostic or assuming the Kerr solution for the black hole. Analysing the statistical properties of these methods in detail for signals similar to GW190521, we find they perform robustly and effectively in distinguishing the presence of multiple quasi-normal modes from noise. We also find that simulated GW190521-like signals with a (3, 3, 0) mode present yield tight  constraints on deviations of that mode from Kerr, whereas constraints on the (2, 2, 1) overtone of the dominant mode yield wide constraints that are not consistent with Kerr. These results on simulated signals are similar to what we find for GW190521. Applying our methods to GW190521, we estimate the redshifted mass and dimensionless spin of the final black hole as ~328 solar masses and ~0.86, respectively. The detection of the two modes disfavours an equal-mass binary; the mass ratio is constrained to 0.4 (+0.2−0.3). We find that the final black hole is consistent with the no-hair theorem and constrain the fractional deviation from general relativity of the sub-dominant mode’s frequency to be −0.008 (+0.08−0.09).

The interaction of Fast Radio Burst (FRB) radio waves with the intervening plasma between us and the sources is viewed, at times, as a nuisance for probing intrinsic FRB properties. At the same time, it offers unique ways to probe interstellar matter at large redshifts and the intervening intergalactic medium. I will show that FRBs are a promising and unique tool for exploring the H reionization history of the Universe. In addition, due to multi-path propagation, a magnetized plasma screen can cause temporal broadening of the FRB, lightcurve variability, spectral decoherence, depolarization, induced circular polarization and image broadening. I will describe how these different properties are directly inter-related, how they manifest in FRB and pulsar observations, how this can already be used to constrain the nature of the intervening plasma in some bursts and how it affects lensing prospects of FRBs.

Hawking radiation is usually defined as radiation expected to be emitted by the event horizon of black holes. This radiation originates from the interaction of the quantum field on the classical background of the curved spacetime around the black hole. This effect is often considered a reliable prediction although it has not been detected and it relies on some dubious assumptions. However, it is possible to view this effect in a more general way. What is a black hole? What are the necessary ingredients to create Hawking radiation? What if instead of gravity we use a different interaction to create a curvature? What type of horizon is necessary? What is the role of negative frequencies in the phenomenon? All these questions are answered by a relatively new and small field of study known as analogue gravity, where effects usually related to gravity are studied in other systems. The most successful ones so far are water tanks, Bose-Einstein condensate, and light pulses in dielectrics. In this talk, we will discuss these topics to lead to a broader understanding of their details, we will present in depth the optical analogues, and we will discuss recent theoretical and numerical results of our research group, as well as some experimental ones in collaboration with laboratories from Mexico and Israel.

Note exceptional day and location: Sunday at Reznick.

Dr. Bermudez is a candidate for a position in the department.

The General Theory of Relativity needs at least one modification - the Cosmological Constant. Yet there are possibilities for other modified theories of gravity to explain the accelerated expansion. In this talk I'm going to discuss the impact of Modified Gravity on the two-body problem. In particular, with the latest observational constraints from the galactic center, binary pulsars and the Milky and Andromeda dynamics.

Dr Benisty is a candidate for a position in the department

Magnetic reconnection is known to transform the magnetic energy into other forms of energy in laboratory and space plasmas. Solar physics and physics of the terrestrial magnetosphere consider this mechanism as the main candidate for acceleration of charged particles to suprathermal energies at reconnecting current sheets. It took scientists several decades to make the way from imaging magnetic reconnection as the simplest Petschek or Sweet-Parker mechanism operating at Harris-type current sheets to understanding this as a 3D turbulent/intermittent/stochastic process associated with creation of flux ropes, secondary current sheets and waves. Studies of magnetic reconnection in the solar wind were far less easy than those in the magnetosphere, first of all, because of the insufficiency of data from spacecraft and the absence of multi-point observations in the heliosphere. As a result, the views on the subject developed in the same way as in magnetospheric physics but it took us longer to realize some critical points about the complexity of magnetic reconnection in heliospheric plasmas. Furthermore, the idea of local particle acceleration by magnetic reconnection in the solar wind has been denied for a long time, and the situation shifted toward its acceptance only during the last decade. Historical and modern views on magnetic reconnection and local particle acceleration in the heliosphere will be discussed in the presentation, with a focus on observations from past and recent heliospheric missions at different distances from the Sun.

Dr. Khabarova is a candidate for a position in the department

The discoveries of thousands of extrasolar planets in our galaxy raise fundamental questions about formation, composition and interior structure of planets. In traditional models of planetary interiors the planets are structured in 2-4 distinct layers of different composition (iron, rock, water, gas), similar to the Earth. However, new theoretical and observational evidence suggest that in general planetary interiors may be very different from this simplified picture. I will show some of these new findings and discuss how we expect planetary interiors to look, based on recent planet formation and evolution models.  

The Standard Model (SM) of particle physics describes all known elementary particles and fundamental forces, excluding gravity. There is, however, growing evidence that the Standard Model is incomplete. Recent years have seen an extensive effort put into finding evidence for interactions beyond the Standard Model (BSM) of physics, making use of precision measurements of nuclear beta-decays in different nuclei. To identify these BSM signatures, experimental measurements must be compared with accurate theoretical predictions. In this talk, I will introduce this experimental surge, motivating me to develop the theoretical formalism required for the analysis of experiments currently being conducted. Presenting a new approach for decomposing tensor interactions of fermionic probes within the multipole analysis, I will examine how signatures of new physics appear in the correlations between the particles emitted in β-decays, and in the energy spectrum of allowed and forbidden decays, which I will show that have an increased sensitivity to these signatures. In addition, to be able to distinguish between new physics signatures and high orders of the known SM physics, I will describe a general framework, suitable for any nucleus and any decay, enabling precise calculations of β-decay observables required for ongoing experiments, with controlled accuracy, evaluated by identifying a hierarchy of small parameters related to the low momentum transfer that characterizes β-decays. First applications to 6He and 23Ne β-decay ongoing measurements at the SARAF accelerator, Israel, will be presented, resulting in new constraints on BSM tensor interactions, and paving the way for new, even more accurate, experiments and discoveries.

The James Webb Telescope has been launched to space using an Ariane 5 rocket. The telescope that promises a leap in our understanding of the universe weighs 6.5 metric tons and is be located at the Lagrange L2 point, 1.5 million km from Earth. How do we deliver something so big so far? We burn hundreds of thousands of kilograms of hydrogen and oxygen.

Combustion is one of the oldest technologies of mankind. The use of fire probably began before the advent of Homo Sapiens, and it is still the main source of energy today. We use combustion for heating, generating electricity, propelling vehicles on land, sea, air and space, and more. The widespread use of combustion has created many opportunities but has also led to many problems.

In this seminar we’ll review the basic principles of combustion and its selected uses will be offered - from heating homes to space flights. Modern challenges of the field will be discussed. A speculation regarding the future of combustion will be presented. No demonstrations are expected due to safety regulations.

About the presenter: Victor is a Senior Lecturer at the Department of Mechanical Engineering, ORT Braude College of Engineering. He holds a Ph.D. in Aerospace Engineering and specializes in rocket propulsion, combustion and fluid mechanics.

Astrophysical black holes are known to be rotating. The simplest spacetime solution describing a classical rotating black hole (the Kerr solution) reveals a non-trivial spacetime structure, in which the geometry connects through an inner horizon to another external universe. But does such a traversable passage really exist inside a physically-realistic spinning black hole?

Answering this question, along others, requires one to understand the manner in which quantum energy fluxes affect the internal geometry of a black hole. It has been widely anticipated, yet inconclusive (till this work), that semiclassical effects would diverge at the inner horizon of a spinning black hole. Such a divergence, if indeed takes place, may drastically affect the internal black hole geometry, potentially preventing the inner horizon traversability. Clarifying this issue requires the computation of the quantum energy fluxes in black hole interiors. However, this has been a serious challenge for decades. 

Using a combination of new and old methods, we have recently managed to compute the semiclassical energy fluxes at the inner horizon of a spinning black hole, in a vacuum state corresponding to an evaporating black hole. We found that these fluxes are either positive or negative, depending on the black hole spin (and polar angle). The sign of these fluxes may be crucial to the nature of their backreaction on the geometry (as should be dictated by the semiclassical Einstein equation).

In this talk, we shall describe the basic framework of semiclassical general relativity and the regularization procedure, and then present our novel results for the semiclassical fluxes at the inner horizon of a rotating black hole, briefly mentioning possible implications for the inner horizon traversability.

An increasing amount of observational data provides us with more and more manifestations of neutron star (NS) oscillations. To interpret existing and future observations one needs to develop adequate models of oscillating NSs. Such models have to account for baryon pairing in stellar interiors. In my talk I will first discuss some general characteristics of oscillation modes in superfluid NSs, their spectra and dissipation timescales. I will then present some more details about two specific classes of oscillations called g-modes, which might excite resonantly during the neutron star inspirals, and about r-modes, which can serve as a powerful tool to probe the properties of superdense matter. Finally, I will consider possible imprints of superfluid oscillation modes in gravitational wave signal from neutron star inspirals.

In order to correctly interpret observations of neutron stars (NSs) and constrain the properties of
superdense matter, it is necessary to have an adequate theory describing the large-scale NS
dynamics. In the first part of the talk I will discuss the key ingredient of such a theory, the
superfluidity of nucleons in the inner layers of NSs, as well as the corresponding
(magneto)hydrodynamic equations. In the second part of the talk, I will briefly review several
possible methods for studying the internal structure of NSs and discuss some problems and
advances associated with these methods.

Some dwarf galaxies like Fornax dwarf Sph containing old globular clusters pose an interesting timing puzzle. The dynamical friction timescales ~ a few Gyrs calculated from the Chandrasekhar formula based on a local theory are too short and imply that these globulars should have already sunk to the center of the host galaxy. This timing problem was solved by N-body simulations which suggest suppression of dynamical friction in cored density profiles of the host. The local theory can not capture this effect and we explore this problem with Tremaine-Weinberg theory which is a global approach and takes into account the real orbital structure of the host galaxy. In this linear response theory, the dynamical friction acts via resonant interactions between perturber and background stars. We show that the number and strength of resonances are reduced inside the core which leads to a suppressed dynamical friction and stalls the evolution of perturber's orbit away from the galaxy center (commonly called "core stalling"). We further explore the density wakes of perturber and find interesting geometrical transformations associated with core stalling.

This talk will be based on these papers:

https://arxiv.org/abs/2112.10801

https://arxiv.org/abs/1810.00369

In the first part of the talk, I will discuss PBH formation, accompanying density induced GWs and probing small scales of inflation. Also I will discuss stellar mass, asteroid mass and super massive BH mass range and their possible interesting implications for cosmology. I will also discuss how we can probe PBHs conclusively via multi messengers (pulsar-time arrays and CMB distortions). In the second part, I will discuss the Fundamental Plane of BH activity (correlation of radio and Xray radiation) and its spin modification, ie the spin influence on accretion and jet processes. I will finally discuss,  we can derive spin bounds for quasars and these spin values could be used to probe/derive bounds for ultra light boson properties via superradiance such as mass, self-interaction and energy density.

This week's Astro Seminar is replaced by the annual Bekenstein Memorial Lecture in Fundamental Physics, this year by 2020 Nobel Laureate Roger Penrose (Oxford). The lecture is organized by the Israel Physics Colloquium (IPC) and by the Racah Institute of the Hebew University (HUJI).

Title: Limits on perturbative treatment for inflationary potentials and what does it mean for the next stage of precision cosmology.

In this talk we briefly discuss the notion of slow-roll inflation, and the mechanism with which it creates the structure of the observed universe. We outline the problem of inferring inflationary potentials from the shape of the primordial power spectrum (PPS), which in itself is a statistical inference from the shape of the matter power spectrum.

We then present some of the methods and heuristics used in building models of inflation, and show that in the age of precision cosmology, they are insufficient. We argue that our current employed statistical and theoretical methods effectively smooth over features in the PPS, which may be crucial for our understanding of high energy physics.

Finally, we comment on the confidence limits of the theoretical methods in model building, and argue that for some classes of inflationary models, no analytic approximation using the current tools may be feasible.

This talk is based on arxiv:2110.10557

Note: Ira is a candidate for the department. Anyone interested in meeting with him can mark themselves here:

https://docs.google.com/spreadsheets/d/1KATnuOMVFMd3kUxxTpiNJhJYWZnZSSY…

The vast majority of detected planets are observed indirectly, using their small perturbation on the light emitted by the host stars. In recent years, however, the world's largest ground based telescopes have succeeded in directly imaging the light coming from some planets themselves. I will present our comprehensive theory for the mass, luminosity, and spin of gas giant planets during their final stages of formation - when they simultaneously contract and accrete gas from a disk. I will apply this theory to the luminosity and spectrum obtained by the novel direct-imaging technique, highlighting the recently discovered PDS 70 system, where two planets were directly observed during formation for the first time.

Note: The seminar will be given *on campus*, and Sivan will be available for discussions during the day for anyone interested. Anyone interested in speaking with him is invited to register for a slot here: https://docs.google.com/spreadsheets/d/1o-xN-B27a_htzTMo-dwdfMmX4mzWfGg…

Black holes are characterized by four externally observable classical parameters: mass, angular momentum, electric and magnetic charges. Recent studies showed that this standard picture is incomplete and that black holes are also characterized by an additional new property - the "magnetic-mass" (or "dual-mass"). The discovery of the magnetic-mass revealed, in turn, a brand new mathematical structure in General Relativity as well as novel observational signatures that I will discuss in detail.

Note: Uri is a candidate for the department. Anyone interested in speaking with him is invited to register for a slot here: https://docs.google.com/spreadsheets/d/1KATnuOMVFMd3kUxxTpiNJhJYWZnZSSY…;

Due to new COVID-related travel restrictions, the seminar will be given over Zoom

The universal law of gravitation has undergone stringent tests for many decades over a significant range of length scales, from atomic to planetary. Of particular interest is the short distance regime, where modifications to Newtonian gravity may arise from axion-like particles or extra dimensions. We have constructed an ultra-sensitive force sensor based on optically-levitated microspheres with a force sensitivity of 10^(−16)N/√Hz to investigate non-Newtonian forces that couple to mass with a characteristic scale of ∼ 10μm. In this talk, I will present the first investigation of the inverse-square law using an optically levitated test mass, along with the technical development that preceded it.

In addition, I will present another precision measurement conducted with the same setup aiming to determine if the charge of the proton is equal in magnitude to the charge of the neutron. This equality has been tested with great precision over the last century and has supporting arguments from the theory side. However, this measurement is a sensitive tool to probe new physics as it is breaking down in a few suggested extensions of the standard model.

Nadav is a candidate for the department; anyone interested in meeting in person for discussions, may fill in the schedule here: https://docs.google.com/spreadsheets/d/1o-xN-B27a_htzTMo-dwdfMmX4mzWfGg…

Shmuel is a candidate for the department.

Note unusual date (Wednesday), because Tuesday is 10th Tevet. The seminar will be *only* on Zoom (meeting id 9290951953) - at 17:00 Jerusalem Time.

The Concordance Model of Cosmology describes the Universe on large scales and its parameters have been measured to an accuracy of a percent.

I will discuss current challenges of the model and attempts to address them. 

Emphasis will be given to the role emergent collective phenomena beyond a single scalar field may have in the early and late Universe. 

As an application I will show that the thermal average of "unparticles" can avoid the Big Bang singularity, act as a Dark Energy model and reduce the Hubble tension.

Note: This seminar will be given *only* on Zoom (meeting id 9290951953), from sunny California.

CARMENES is a radial-velocity (RV) survey for exoplanets around nearby M dwarf stars. Using a high-resolution dual-channel spectrograph, it provides M-star RV measurements with a precision down to ~1 m/s. In five years of surveying ~300 nearby M-dwarfs, we have detected ~30 new planets, including habitable-zone Earth-mass planets (Teegarden’s Star b&c) and planets that challenge planet-formation models. Some of the new planets are amenable for characterization by next-decade direct-imaging and astrometric instruments. In addition, CARMENES was used to estimate the masses of ~12 transiting planets around nearby M dwarfs, including AU Mic b&c. CARMENES’ unique design allows addressing additional questions related to M dwarfs and close-in planets, such as the spectroscopic manifestation of photospheric activity and rotation, magnetic field strength of active stars, and the atmospheric composition of hot massive planets. In the talk, I will give a brief overview of the latest results from CARMENES and an outlook to its future.

Interaction-powered supernovae (SNe) explode within an optically thick circumstellar medium (CSM) that could be ejected during eruptive events. To identify and characterize such pre-explosion outbursts, we produce forced-photometry light curves for 196 interacting SNe, mostly of Type IIn, detected by the Zwicky Transient Facility between early 2018 and 2020 June. Extensive tests demonstrate that we only expect a few false detections among the 70,000 analyzed pre-explosion images after applying quality cuts and bias corrections. We detect precursor eruptions prior to 18 Type IIn SNe and prior to the Type Ibn SN 2019uo. Precursors become brighter and more frequent in the last months before the SN and month-long outbursts brighter than magnitude −13 occur prior to 25% (5–69%, 95% confidence range) of all Type IIn SNe within the final three months before the explosion. With radiative energies of up to 1049 erg, precursors could eject  ∼1 M ⊙ of material. Nevertheless, SNe with detected precursors are not significantly more luminous than other SNe IIn, and the characteristic narrow hydrogen lines in their spectra typically originate from earlier, undetected mass-loss events. The long precursor durations require ongoing energy injection, and they could, for example, be powered by interaction or by a continuum-driven wind. Instabilities during the neon- and oxygen-burning phases are predicted to launch precursors in the final years to months before the explosion; however, the brightest precursor is 100 times more energetic than anticipated.

The region of spacetime near the event horizon of a black hole can be viewed as a deep potential well at large gravitational redshift relative to distant observers. However, matter orbiting in this region travels at relativistic speeds and can impart a significant Doppler shift to its electromagnetic emission, sometimes resulting in a net observed blueshift at infinity. Thus, a black hole broadens the line emission from monochromatic sources in its vicinity into a smoothly decaying “red wing”—whose flux vanishes at large redshift—together with a “blue blade” that retains finite flux up to a sharp edge corresponding to the maximum observable blueshift. In the talk, I will describe the blue blade produced by isotropic monochromatic emitters on circular equatorial orbits around a Kerr black hole, and outline how the maximum blueshift simply encodes black hole spin and inclination. These results bear direct relevance to ongoing and future observations aiming to infer the angular momenta of supermassive black holes from the broadening of their surrounding line emission.

The standard cosmological model, which is firmly based on General Relativity (GR), has been very successful in parametrically fitting diverse combinations of observational datasets. This success rests on the stipulated existence of dark matter (DM) and dark energy, both of which remain elusive. In addition, the model heralds the breakdown of our basic concepts of space and time at the initial Big Bang singularity.

In this talk I will argue that extending the symmetry of GR to accommodate Weyl-invariance (WI), i.e. allowing for our fundamental measure sticks, such as the Planck length, to vary in space and time could potentially obviate the need for clustering DM on all scales, avoid the initial singularity problem with a bouncing model of the Universe, and resolve several other puzzles afflicting the standard cosmological model. 

Note: Meir is a candidate for the department

When massive stars exhaust their nuclear fuel, their core collapses to form a compact object, releasing a large amount of energy and giving rise to an outward moving shock wave. If this shock wave is able to overcome gravity, the star explodes and produces a transient called a supernova. If, instead, gravity overwhelms the shock wave, then the star collapses directly to a black hole. In this talk I will discuss my research of the passage of this shock wave inside the interior of the star. I will focus on two extremes: the behaviour close to the centre of the star (the explosion mechanism) and near the stellar surface (shock breakout). I will show how results from this study, with the data from upcoming missions, can shed light on the properties of the progenitor star and the outcome of the star’s death (whether it explodes or not, and what kind of object it leaves behind). I will also discuss how insights from this study can be used to model other violent astrophysical processes.

Note: the seminar will be given remotely over Zoom, on https://zoom.us/j/9290951953

Faithful, robust and fast waveform models are of critical importance to gravitational wave (GW) astronomy to allow for accurate and precise detection and analysis of the source. Waveform models based on the Effective-One-Body (EOB) approach have been proven to be very powerful in their ability to combine analytical information from PN theory, gravitational-self-force theory and more, in order to capture the full picture of merging binary systems. Purely analytical EOB models are however still of insufficient quality to be used in the detection and analysis of GW events. This thesis presents an introduction to the solution of this problem: The completion of EOB waveform models through Numerical Relativity (NR), on the example of non-precessing, non-eccentric Binary Black Hole (BBH) systems, utilizing the framework of the TEOB model. Once completed NR is further used to validate the model to ensure it meets the qualitative needs of GW data analysis.

The infrastructure of the TEOB model is introduced and discussed with a strong focus onto analytical flexibilities that can be used to capture missing information from NR waveforms. The analytical flexibilities of the TEOB model are made up of effective parameters that enter the Hamiltonian so as to modify both the orbital part (i.e.~non-spinning) and the spin-orbit interaction between the orbital angular momentum and the black hole spins. The approximation of a quasi-circular inspiral is corrected effectively in the radiation reaction of the system by imposing NR fitted waveform characteristics. The model is completed with a phenomenological template fitted directly to NR to capture the merger and ringdown of the BBH system. In total 154 BBH-NR waveforms are combined to inform the TEOB. An additional 460 waveforms are used to validate the model. These waveforms span over a large part of the parameter space reaching mass-ratios $m_1/m_2\leq 18$ and black hole spins of up to $|\vec{S}_{1,2}|/m^2_{1,2} \leq 0.998$. This calibration process is presented for three, successively improving avatars of the TEOB model. The TEOB avatars discussed in this thesis are: Firstly, TEOBResumS is a model for the dominant, quadrupolar mode; Secondly, TEOBiResum_SM models BBH systems of non-rotating black holes, extending the calibration of the quadrupolar mode to a large set of 9 further subdominant modes; Finally, TEOBiResumS_SM extends the calibration of all but one subdominant mode to the full spin-range available of available NR waveforms. The fully calibrated models are all evaluated against the NR catalog. In many instances the model does not just meet but exceeds the quality demands for application in GW astronomy. 

The talk will be given on Zoom: https://zoom.us/j/9290951953 at 17:00 Israel Time (16:00 CEST).

With the release of the second Gravitational Wave Transient Catalog (GWTC-2), there are now nearly 50 confident compact binary mergers detected by the LIGO and Virgo instruments.  This includes multiple detections consistent with the presence of a neutron star.  Whereas the first such detection, GW170817, was confirmed to contain at least one neutron star by its electromagnetic counterpart, none of these new candidates have counterparts to aid their classification.  GW190814 is of particularly ambiguous origins, as its smaller compact object (2.50-2.67 solar masses at the 90% credible level) is either the largest known neutron star, or the smallest known black hole.

While most previous population studies focus in on a single source category (most frequently binary black holes), the presence of at least one ambiguous event makes it necessary to simultaneously fit all three source categories (binary black holes, binary neutron stars, and neutron star-black hole binaries).  In this talk I will discuss several recent analyses which do precisely this.  I also apply the same techniques to identify subpopulations within the broader binary black hole population.

The apparent missing mass in galaxies and galaxy clusters, commonly viewed as evidence for dark matter, could possibly originate from gradients in the gravitational coupling parameter, $G$, and active gravitational mass, $M_{act}$, rather than hypothetical beyond-the-standard-model particles. We argue that in (the weak field limit of) a Weyl-invariant extension of General Relativity, one can simply affect the change $\Phi_{b}(x)\rightarrow\Phi_{b}(x) + \Phi_{DM}(x)$, where $\Phi_{b}$ is the baryon-sourced potential and $\Phi_{DM}$ is the `excess' potential. This is compensated by gradients of $GM_{act}$ and a fractional increase of $O(-4\Phi_{DM}(x))$ in the baryon density, well below current detection thresholds on all relevant scales.

Evidence for echo signals from black holes would be a phenomenal indication of new physics beyond standard gravitational models. I will discuss some of the wide range of tests that have been performed to date on gravitational wave data and discuss some of the theoretical and observational challenges as we go forwards. 

The three-body problem in Newtonian gravity is one of the oldest and richest problems in physics. Giants have worked on it and it has been the source of numerous fields in theoretical physics and mathematics including perturbation theory, topology and chaos. Yet, it remains unsolved, and the associated statistical theory remains incomplete and flawed even after almost fifty years of work. Inspired by recent beautiful work of Nick Stone and his collaborator Leigh, I have developed a reduction of the outcome probability distribution. In this sense, I believe the problem has been cracked, as will described in the talk.

The talk is based on https://arxiv.org/abs/2002.11496

Fast and accurate binary-black-hole (BBH) merger waveform models that span wide parameter ranges are crucial for future searches and parameter estimation of gravitational-wave data. To date, analytical waveforms that incorporate numerical-relativity information, such as effective-one-body and phenomenological models, play an important role in analysing LIGO and Virgo data. However, these models are not automatically updated every time new numerical waveforms become available. Here we present a new perspective on dynamically tuning waveform models by incorporating sparse information from a more accurate model. We also show the first attempts to use our method to include additional physical effects that were not present in the original model and investigate various techniques that include interpolation and regression implemented in the development of waveform modeling.

 

The talk will be given over Zoom, at https://zoom.us/j/9290951953

Vassilios Mewes

National Center for Computational Sciences and Physics Division, Oak Ridge National Laboratory

The new era of gravitational wave multi-messenger astrophysics began with the recent detection of the binary neutron star merger GW170817. Our theoretical understanding of these systems relies on high fidelity numerical relativity simulations including general relativistic magnetohydrodynamics, realistic equations of state for matter up to nuclear densities, and neutrino radiation hydrodynamics. The approximate symmetries of the post-merger stage of the evolution, namely hypermassive neutron stars and black hole torus systems, make spherical coordinates better suited than Cartesian coordinates for the numerical modelling of these systems. This seminar will present SphericalNR, a new framework within the publicly available Einstein Toolkit to numerically solve the Einstein field equations of general relativity coupled to the equations of general relativistic magnetohydrodynamics in spherical coordinates without symmetry assumptions. A description of a reference metric approach together with algorithmic details enabling the use of spherical coordinates in the originally Cartesian code base of the Einstein Toolkit will be presented, followed by a description of ongoing algorithmic and code development work regarding a double FFT filter with the aim to alleviate the extremely severe timestep restrictions when solving hyperbolic PDEs in spherical coordinates with high angular resolutions. The outlook will touch upon future development for SphericalNR, focusing on extending the multi-physics capabilities of the framework, as well as challenges for increasing the parallel efficiency of the code with a view on the upcoming exascale era of HPC.

In this talk I will describe a phenomenon akin to Electric-Magnetic duality in Einstein's gravity. I will show that a new type of "magnetic" dual gravitational charges generate redundant symmetry transformations which are not part of the standard group of diffeomorphisms. General Relativity is therefore shown to possess an additional gauge symmetry of the metric which reveals, in turn, a wide class of new IR phenomena.

Space weather affects life on Earth and in outer space. Human technologies are affected by coronal mass ejections and similar outbursts of solar activities. Accurate prediction of the solar wind and its polarity can help understand the Sun and its dynamic environment. The Wang-Sheeley-Arge (WSA) phenomenological model of the coronal magnetic field can estimate solar-wind speed and interplanetary magnetic field polarity in the inner heliosphere by using photospheric magnetic field maps. WSA has historically used two parameters, the source surface and interface radii, to tune its predictions. In this talk, I describe how our team used sequential Monte Carlo, also called particle filtering, in the assimilation of satellite data to adjust the values of these radii. Adaptive optimization, applied to week-long timescales across several months of historical data, yielded approximately double predictive performance. In addition to improved forecasts, this statistical study highlights challenges in parameter estimation for the nearest and most-observed solar-mass object: the Sun.

*Approved for Los Alamos Unlimited Release: LA-UR-21-21918.

In this talk I will talk about a recent work where we propose a new test of GR. The gravitational waves emitted during the coalescence of binary black holes offers an excellent probe to test the behaviour of strong gravity at different length scales. In this work, we propose a test called the merger-ringdown consistency test that focuses on probing horizon-scale dynamics of strong-gravity using the binary black hole ringdowns. This test is a modification of the more traditional inspiral-merger-ringdown consistency test. I will present a proof-of-concept study of this test using simulated binary black hole ringdowns embedded in the Einstein Telescope-like noise. Furthermore, we use a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.

The talk is at 17:00 Jerusalem Time (16:00 CET), on Zoom: https://zoom.us/j/9290951953  

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

From the assumption that the slow roll parameter ϵ has a Lorentzian form as a function of the e-folds number N, a successful model of a quintessential inflation is obtained, as succinctly studied in \cite{Benisty:2020xqm}. The form corresponds to the vacuum energy both in the inflationary and in the dark energy epochs and satisfies the condition to climb from small values of ϵ to 1 at the end of the inflationary epoch. We find the corresponding scalar Quintessential Inflationary potential with two flat regions. Moreover, a reheating mechanism is suggested with numerical estimation for the homogeneous evolution of the universe. The suggested mechanism is consistent with the BBN bound.

Based on: https://arxiv.org/abs/2006.04129

The talk will be given on Zoom:

We will meet about 30 minutes before the official start time for virtual mingling and informal chat

We have entered a golden age for studying astrophysical “transients” (short and violent astrophysical events). Recent advances in the field of large-scale, high-cadence astronomical surveys have provided unprecedentedly rich and diverse data sets and transient astronomy is said to have entered a “big data era”. I will give a short overview of several on-going and future instruments which are real « game-changers » in the field and will show how combining their assets, as well as using tools from  "data science" - involving high-performance computing and data mining - can lead to promising new science. I will then share recent results from the first UV survey of the early evolution of interacting supernovae and from mining twenty years of X-ray archival data in search for the shortest transients, including the elusive signatures of the mysterious supernova "shock breakout".

Note the unusual date/time (Wednesday morning). 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

In this talk, we discuss the exciting possibilities of exploring strong gravity with future GW observations. The properties of Black-Holes (BH) and exotic compact objects (ECOs) immersed in a tidal environment are discussed. In particular, we focus on the BH reaction and its induced quadrupole moment to an external tidal field. The no-Love number theorem for GR BHs is thoroughly reviewed and alternative explanations are suggested. Here, I'll present for the first time how simple quantum mechanical arguments support the existence of the Love number in quantum black-holes, moreover I'll show how the detection of these quantum induced effects is possible with future precision gravitational wave measurement.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The start time is 17:00 Jerusalem time

Several billion years into the future, our Sun will break away from the main sequence. It would expand, turning into a giant star, eventually shedding its outer layers to become a white dwarf -- a perpetually fading remnant of its former glory. We will not be around to see this. Planet Earth will be engulfed in the process. In the outer Solar system, planets and minor planets will be baked by the intense radiation from the giant Star, becoming active as comets do. Their orbits will expand, and this would give rise to rich dynamical interactions. In the aftermath of this calamity, many surviving objects would be injected into tidal crossing orbits of our Sun's ultra-dense successor, the white dwarf. As they do, they will be violently and repeatedly ripped apart, breaking into their smallest constituent building blocks. While we cannot hope to glimpse our own future, nature has given us a unique
opportunity to triumphantly jubilate as we watch the demise of other, less fortunate exo-planetary systems. In my talk I would briefly discuss various topics related to white dwarf atmospheric pollution by exo-planetary remnants: focusing on the properties and chemistry of the polluters ; the formation of debris discs and compact accretion discs; and the growing number of recent discoveries of individual (disintegrating or intact) minor and major exo-planets observed in orbit of white dwarfs.

The talk will be give over Zoom, at https://zoom.us/j/9290951953

Marginally outer trapped surfaces (MOTSs) are the main tool in numerical relativity to infer properties of black holes in simulations of dynamical systems. In this talk, I will present results that show how we can understand a merger of two black holes in terms of the evolution of these MOTSs. This closes a gap in our understanding of binary-black-hole mergers and provides the quasilocal analog of the famous "pair-of-pants" picture of the event horizon of two merging black holes. In particular, we will encounter three new phenomena: (i) the merger of MOTSs, (ii) the formation of self-intersecting MOTSs immediately after this merger, and (iii) a non-monotonicity result for the area of certain smoothly evolving MOTSs. Finally, I will show a remarkable correspondence between the evolution of the horizon geometry at late times and the quasi-normal modes which describe the ringdown signal measurable by far away observers.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

Abstract: In the presence of a black hole, light sources connect to observers along multiple paths. As a result, observed brightness fluctuations must be correlated across different times and positions in black hole images. Photons that execute multiple orbits around the black hole appear near a critical curve in the observer sky, giving rise to the photon ring. In the talk I will describe the structure of a Kerr black hole's photon ring. I will then discuss a novel observable we have recently proposed: the two-point correlation function of intensity fluctuations on the ring. This two-point function exhibits a universal, self-similar pattern consisting of multiple peaks of identical shape: while the profile of each peak encodes statistical properties of fluctuations in the source, the locations and heights of the peaks are determined purely by the black hole parameters. Measuring these peaks would demonstrate the existence of the photon ring without resolving its thickness, and would provide estimates of black hole mass and spin. With regular monitoring over sufficiently long timescales, this measurement could be possible via interferometric imaging with modest improvements to the Event Horizon Telescope.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

A new astronomy started in 2015 with the first gravitational wave detection. Since then, LIGO-Virgo collaborations have confirmed several
events originating from the coalescence of binary systems composed by compact objects. While for these type of events the wave signature is
clear and well characterized, for other possible sources in the universe we are lacking of a complete modelization, so that it is necessary to
adopt alternative approaches to discover. We overview one of these approaches extensively used in the search for gravitational wave in the
LIGO-Virgo scientific runs, exploting its performances and possible applications. We also introduces the future challenges, when the next generation of detectors will become operative.

*Incidentally, Marco Drago was the first person to see a gravitational wave - and he'll be available to chat before and after the talk

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

Many astrophysical phenomena involve an abrupt release of a large amount of energy close to the surface of a large body. Examples include impacts on a terrestrial planet and outbursts from a neutron star while inside the common envelope of a giant star. In this talk I will present a new universal analytic solution that can describe the shock wave in all these scenarios. I will show that this shock wave satisfies a new kind of conservation law that lies somewhere in between energy and momentum conservation. This conservation law opens the door to a myriad of insights about a wide range of physical problems: the size and shapes of craters, atmospheric mass loss from giant impacts and oblique shock breakout from supernovae.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for virtual mingling and informal chat

The recent detection of gravitational waves (GW) from a system of binary neutron stars (BNS) in coincidence with electromagnetic observations has launched a new era of multimessenger astrophysics. As a result, BNS mergers are one of the main targets for GW interferometer detectors on earth. A particularly interesting challenge is to constraint the equation of state (EOS) of the nuclear matter inside the neutron star core, which is still theoretically unknown. In order to do parameter estimation and detect additional GW signals, we need to compare the observed signals to theoretical GW templates, which depend on different characteristics like total mass, EOS, mass ratio, etc. Limited work has been previously done with simulating unequal-mass BNS because of numerical difficulties. We have modified the LORENE code to advance our ability to construct unequal-mass BNS initial data, and used them to initiate dynamical evolutions of BNS mergers performed using the Einstein Toolkit. Here we discuss the importance of Initial Data and the modifications done to the LORENE code. 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

Black Widows are rapidly spinning magnetized neutron stars with companions that are only a few percent the mass of the Sun. I will present numerical stellar evolution tracks showing how main sequence stars are reduced to such low masses by magnetic braking and Roche-lobe overflow. The numerical results are explained by an analytical model, similar to the Hayashi track, but accounting for the pulsar’s gamma-ray irradiation. I will compare the theory to radio and gamma-ray observations of the pulsars, as well as to novel optical images of the companions themselves. I will demonstrate that the mass at which a Black Widow companion becomes fully convective is a simple function of its orbital period, allowing us to study stellar structure and magnetism away from the main sequence in a controlled manner. 

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

The equation-of-state of cold asymmetric (neutron-rich) matter at supra-nuclear densities is a long-standing open question. It cannot currently be calculated from first principles theory (QCD), nor can this regime be directly probed through terrestrial experiments. Neutron stars (NSs) therefore provide a unique laboratory for studying cold dense matter. In this talk I will describe progress in this field obtained by utilizing both gravitational-wave and electromagnetic observations of NS mergers. An overview will be given of several different methods of constraining the equation-of-state using NS mergers, the strengths and caveats associated with each will briefly be discussed, as will the future outlook for this newly-emerging field.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for informal chat

Gravitational wave observations can now strongly differentiate between assumptions for how binary compact objects form. Different models for compact binary formation can be ranked by their similarity to GW observations, as a marginal likelihood. In this work, we show how to carefully interpolate this marginal likelihood between model parameters, enabling posterior distributions for these model parameters. Using the StarTrack binary evolution code, we compare one- and three-dimensional models to the compact binary mergers reported in GWTC-1. Consistent with prior work, with our one- dimensional models we infer that modest natal kicks are more consistent with the observed merger rates and mass distributions.

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Tel-Aviv time, but we'll start gathering 30 minutes earlier for informal chat

Note: Israel switches to Winter Time on October 25th, and the US on November 1st, so 17:00 in Tel-Aviv means 11:00 in Rochester

*** Recording available here: https://www.youtube.com/watch?v=qwhaQQBQY4c&feature=youtu.be ***

I will start with a broad review of the field of star formation and galaxy evolution, and some pressing open questions. I will then dive into the star-forming interstellar medium (ISM), asking the question, what regulates the star formation process?

I will discuss the multiphase structure of the ISM, key heating-cooling processes and chemical processes in the ISM, and interstellar turbulence, all of which may play an important role in regulating star formation. I will focus on a particularly appealing theory for star formation where gas heating by far-UV radiation from young stars (and by cosmic-rays in some galaxies), may provide a natural feedback loop, and thus organically self-regulate star-formation in galactic disks, and present recent results (Bialy 2020, ApJ accepted) for the link between star-formation rate and far-UV radiation intensity.

I will conclude with future prospects: Charting new ways for constraining poorly known interstellar properties: turbulence, 3D ISM structure, low energy cosmic-ray spectra, and our plan to construct an improved star-formation model for next-generation large scale cosmological simulations (i.e., IllustrisTNG successors).

The seminar will be hosted on Zoom: https://zoom.us/j/9290951953

The talk's start time is 17:00 Jerusalem time, but we'll start gathering 30 minutes earlier for virtual mingling and informal chat

PhD Thesis Synopsis for Srishti Tiwari, our incoming postdoc

Zoom link: https://zoom.us/j/9290951953 

Black widows are millisecond pulsars with low-mass companions (~2% the mass of the sun) on short orbits of several hours. When the first black widow was discovered in 1988, it was proposed that its companion is the remnant of a main sequence star that had been evaporated by the pulsar’s high energy radiation. I will present new observations from the last decade that challenge this picture, and discuss how the growing population of black widows can be explained consistently.

The talk will be given over Zoom, at: https://zoom.us/j/9290951953

The official talk starts at 16:30 Jerusalem Time (06:30 PDT), but we'll start preparing and mingling on Zoom from 16:00 and onwards

As two neutron stars merge, they emit gravitational waves that can potentially be detected by earth bound detectors. Matched-filtering based algorithms have traditionally been used to extract quiet signals embedded in noise. We introduce a novel neural-network based machine learning algorithm that uses time series strain data from gravitational-wave detectors to detect signals from non-spinning binary neutron star mergers. For the Advanced LIGO design sensitivity, our network has an average sensitive distance of 130 Mpc at a false-alarm rate of 10 per month. Compared to other state-of-the-art machine learning algorithms, we find an improvement by a factor of 6 in sensitivity to signals with signal-to-noise ratio below 25. However, this approach is not yet competitive with traditional matched-filtering based methods. A conservative estimate indicates that our algorithm introduces on average 10.2 s of latency between signal arrival and generating an alert. We give an exact description of our testing procedure, which can not only be applied to machine learning based algorithms but all other search algorithms as well. We thereby improve the ability to compare machine learning and classical searches.

The seminar will be given online via Zoom: https://zoom.us/j/9290951953

The official talk time is 16:30 Jerusalem Time (15:30 CEST), but we'll be around from 16:00 (15:00) for connection tests and chatter

I will discuss the possibility and feasibility of time travel within the context of general relativity and quantum field theory, the paradoxes resulting from it, and possible ways to resolve these paradoxes. The talk will be based on arXiv:1907.04178 and arXiv:1911.11590.

The talk will be given over Zoom, at: https://zoom.us/j/9290951953

The official talk starts at 16:30 Jerusalem Time (09:30 EDT), but we'll hstart mingling on Zoom from 16:00 and onwards

The first Gravitational Wave detection of GW150914 led to a revolution in the world of modern physics and astronomy,

beyond being an additional confirmation for the predictions of Einstein's General Relativity,

it opened the gates of Experimental Physics to data that couldn't have been observed in any other way before that,

and capable of testing theories that before the GW era had no other way of being proved.

In my talk, i'll analyze some of the LIGO results from a different point of view respect to the one commonly adopted by LIGO,

by looking for statistically significant correlations in the data between the LIGO gravitational wave detectors.

Said method, even though wouldn't be the best choice for a blind search of new gravitational wave events, may be used to infer  properties of already known detections,

as well as testing deviation of the data from the prediction of standard theories and test out new ones.

The seminar will be given over Zoom, at: https://zoom.us/j/9290951953

The time (16:30) is In Jerusalem Time (15:30 CEST)

The immediate surroundings of white dwarfs (WDs) are key to our understanding of a number of puzzles. Observations of WDs can reveal the presence of stellar, substellar, and stellar-remnant companions, planets, dust, atmospheric heavy elements, and planetary debris, each of relevance to several important questions. The remains of the pre-WD-phase solar systems are revealed in the form of heavy element 'pollution' in WD atmospheres, excess emission from dust discs, and–only recently–in transits of planetary debris. In principle, WDs can host not only debris, but also whole planetary systems. Binary systems consisting of two WDs are important in a broad range of astrophysical contexts, from stellar evolution, through Type-Ia supernova (SN Ia) progenitors, to sources of gravitational waves.

SNe Ia–supernova explosions of WDs–are a major source of heavy elements, and, as 'standard candles', they have provided one of the fundamental methods for estimating distances in the Universe. However, the nature of the progenitor systems of SNe Ia is still unclear. A progenitor scenario that has been long considered is the double-degenerate scenario, in which a double WD binary loses energy and angular momentum to gravitational waves, until merger and possible explosion as a SN Ia. If most SN Ia explosions are the result of double WD mergers, then the observed double WD merger rate should be high enough to account for the observed SN Ia rate.

In my talk I will present some of the clues we have found for these questions.

The talk will be given over Zoom, meeting link is  https://zoom.us/j/9290951953&nbsp;

I will start by motivating why some observational probes of astrophysical black holes in a quantum theory might be radically different from their classical ones. I will then show that these signatures can be best probed by searching for low frequency harmonics in the gravitational wave spectrum of perturbed black holes, what we call "quantum black hole seismology". Finally, I will end by summarizing the (controversial) observational status of these searches and their future outlook. 

The seminar will be given over Zoom, at: https://zoom.us/j/9290951953

The time (16:30) is In Jerusalem Time (09:30 EDT)

The Event Horizon Telescope image of the supermassive black hole in the galaxy M87 is dominated by a bright, unresolved ring. General relativity predicts that embedded within this image lies a thin “photon ring,” which is composed of an infinite sequence of self-similar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole “shadow,” becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high order subrings. In the talk, I will discuss the structure of the photon ring, starting with non-rotating black holes, and then proceed to the complex patterns that emerge when rotation is taken into account. Subsequently I will argue that the subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array.

The talk will be given over Zoom, meeting link is  https://zoom.us/j/9290951953 

We study the emission of gravitational waves, gravitons, photons and neutrinos from a perturbed Schwarzschild blackhole (BH).The perturbation can be due to either classical or quantum sources and therefore the injected energy can be either positive or negative.The emission can be classical in nature, as in the case of gravitational waves, or of quantum nature, for gravitons and the additional fields. We first setup the theoretical framework for calculating the emission by treating the case of a minimally coupled scalar field and then present the results for the other fields. We perform the calculations in the horizon-locking gauge in which the BH horizonis deformed, following similar calculations of tidal deformations of BH horizons.The classical emission can be interpreted as due to a partial exposure of a nonempty BH interior, while the quantum emission can be interpreted as an increased Hawking radiation flux due to the partial exposure of the BH interior. At the end we demonstrate that the quantum emission in BHs that are far away from equilibrium is comparable and even larger than the Hawking radiation

Exomoons orbiting terrestrial or super-terrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. I will present results from a recent study about the collision-formation of massive exomoons, and discuss the plausibility of detecting them currently or in the future. We are also able to infer the existence of exo-solar moons, dwarf and minor planets from the observation of polluted white dwarf atmospheres, which probe their bulk composition. Small planetary objects with tidal crossing orbits disrupt around the white dwarf and form a disk of debris, which later accrete onto the white dwarf and pollute its atmosphere. Progress in the last decade, has shown this material to be typically dry, i.e., with terrestrial-like chemical composition and lacking in water. l will discuss results from a series of studies which examine whether water-bearing small planetary objects even have the potential to retain their water, as they undergo thermal, physical, chemical and orbital evolution during the high luminosity stellar evolution phases of their host stars. If time permits I will talk briefly about new hybrid approaches for modeling the aforementioned tidal disruptions and generating debris disks.

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Cosmic-rays are one of the most fascinating phenomena in the universe. They consist of energetic particles with an out-of-equilibrium power-law spectrum extending over at least eleven orders of magnitude in energy, from ~1 GeV to 10^11 GeV. In the past decade, new measurements by experiments such as the Pierre Auger observatory and Telescope Array, have greatly improved our knowledge of the highest energy domain of the cosmic-ray spectrum, the "ultra-high energy cosmic-rays" (UHECR), with energies > 10^9 GeV. At these energies, cosmic-rays are thought to be of extragalactic origin and they are highly challenging by questions with respect to their origins and their acceleration processes. 

I will first review the observational data on the cosmic-ray spectrum, composition and arrival directions. I will show that the spectrum and composition can be explained by a generic model having one Galactic component and one extragalactic component. I will review the multi-messenger constrains brought by neutrino and gamma-ray experiments on UHECR origin. Finally, I will discuss the origin of the UHECR dipole anisotropy recently reported by the Pierre Auger Observatory.

Introducing the physics of gravitational waves and compact binary coalescences, and their detection and analysis by LIGO. Focusing on the catalog from LIGO's first 2 Observation runs O1+O2, as well as engagement opportunities for new students and researchers towards O3, and the future of LIGO and next generation detectors.

The most fundamental question in observational cosmology today is what is the nature of dark energy and dark matter. As the most massive gravitationally bound bodies in the Universe, clusters of galaxies serve as beacons to the growth of structure over cosmic scales, making them a sensitive cosmological tool. However, accurately measuring their masses has been notoriously difficult. Weak lensing provides the best direct probe of the cluster mass, both the baryonic and dark components, but it requires high-quality wide-field imaging. With its unprecedentedly deep and exquisite seeing, the Subaru Hyper Suprime-Cam (HSC) survey is an ongoing campaign to observe 1,400 square degrees. In this talk, I will present our new field-leading results from the first HSC data release of ~150 square degrees that encompass thousands of clusters. Harnessing our new HSC survey, I measure benchmark weak lensing cluster masses, and reconcile previous tension on cosmological parameters between the SZ and CMB within the Planck survey. The next generation of wide-field surveys is almost upon us, with the Large Synoptic Survey Telescope (LSST), WFIRST and several more coming online. They will discover hundreds of thousands of galaxy clusters, peering deep to the epoch of formation. I will describe these exciting new surveys and the multifold breakthrough science we will achieve in the new era of astronomy.

Exoplanets are almost never visible and thus remained unknown over centuries of astronomical research.  In this talk, I will explain how exciting discoveries of new worlds are now made, and surprising aspects of their characteristics are determined. This is accomplished by creative methods and dedicated telescopes on Earth and in space.  I will review the observational techniques for studying exoplanets and focus on transits – the passage of an exoplanet in front of its host star.  This seemingly simple geometry allows a surprising array of insights: from detailed transit analysis, we constrain the most fundamental planetary properties relevant for the system architecture, theories of planet formation, evolution, composition, global weather patterns, and some day, even biomarkers.  The relentless pace of discovery during the past two decades is expected not only to continue but even intensify in the future.

Over the last three decades, our knowledge about planetary systems has increased dramatically, from one example with eight planets (our own Solar system) to over 2800 planetary systems hosting more than 3700 planets. While occurrence rate studies show that exoplanets are the rule rather than an exception, our understanding of the physical processes forming these planets is still very limited. Fortunately, we are now on the verge of the next revolution in exoplanet science. TESS, PLATO, JWST, WFIRST, and LSST will complete the demographic census of planets across a wide range of environments, and will allow detailed characterization of their atmospheres and structure.

   In this talk I will discuss the important role of microlensing in the forefront of exoplanetary studies. Gravitational microlensing is unique in its ability to probe several important but relatively untapped reservoirs of exoplanet parameter space, including the abundance and mass-function of cold planets, planet-formation efficiency in different Galactic environments, and the population of free-floating planets. A wealth of new and upcoming microlensing campaigns, both from ground and space, will allow the full exploration of the exoplanet demographics unique to microlensing, potentially revolutionizing our understanding of planet formation. In addition to studying planets, these surveys allow to study important regimes of the stellar mass function (e.g., massive remnants, isolated brown-dwarfs) and to to study the Galactic structure and evolution.