AstroPhysics Seminar

Tell a Friend
Usual Time
Sunday 10:30
Physics (Building 202), Room 301
Upcoming Lectures
- CARMENES: a radial velocity search for planets around M dwarfs Lev Tal Or, Ariel
Lev Tal Or, Ariel


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 three years of surveying ~325 nearby M-dwarfs, we have detected about a dozen of new planets, including habitable-zone Earth-mass planets and planets that challenge planet-formation models. Some of the new planets are amenable for characterization by next-decade direct-imaging and astrometric instruments. 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 Jupiters. I will give a brief overview of the latest results from CARMENES. If time is left I will comment on a simple way of correcting for systematic errors in large RV surveys.


- Strong Gravity Tests with Tidal Effects Yotam Sherf, Ben-Gurion University
Yotam Sherf, Ben-Gurion University

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 the applied tidal field. The no-Love number theorem for GR BHs is thoroughly reviewed and alternative explanations are suggested. At the end, we study both quantum and classcial emission in GR BHs. 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.

Previous Lectures
Black Widow Evolution Sivan Ginzburg, Berkeley
Sivan Ginzburg, Berkeley

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:

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

Detection of gravitational-wave signals from binary neutron star mergers using machine learning Marlin B. Schäfer, Albert Einstein Institute (AEI) Hannover, Germany
Marlin B. Schäfer, Albert Einstein Institute (AEI) Hannover, Germany

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:

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


Time Travel and Its Paradoxes Barak Shoshany, Perimeter Institute of Theoretical Physics
Barak Shoshany, Perimeter Institute of Theoretical Physics

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:

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

Searching for unexpected signals in the LIGO gravitational wave events Paolo Marcoccia, University of Stavanger, Norway
Paolo Marcoccia, University of Stavanger, Norway

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:

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

- Companions and debris around white dwarfs Na'ama Hallakoun, Weizmann Institute of Science
Na'ama Hallakoun, Weizmann Institute of Science

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 

Quantum Black Hole Seismology Niayesh Afshordi
Niayesh Afshordi

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:

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

Universal signatures of a black hole’s photon ring Shahar Hadar, Harvard
Shahar Hadar, Harvard

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 

- Emission channels from perturbed black-holes Yotam Sherf, Ben-Gurion University
Yotam Sherf, Ben-Gurion University

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

- How (and what) can we learn about exo-solar moons, dwarf and minor planets Uri Malamud
Uri Malamud

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.

- Virialization of Gas in Dark Matter Halos and its Implications for Galaxy Evolution Dr. Jonathan Stern
Dr. Jonathan Stern

Gas in dark matter halos, known as the circumgalactic medium (CGM), is both the source of fuel for star formation in the galaxy and the destination of galaxy outflows. The CGM is thus expected to play a fundamental role in galaxy evolution. I will revisit the question of the virialization of the CGM, in which the volume-filling gas phase transitions from being predominantly cool and free-falling at low halo masses to predominantly hot and quasi-static at high halo masses. Using both an idealized model and the FIRE cosmological simulations, I will demonstrate that several aspects of this process have not been previously appreciated despite over four decades of research. I will then show that CGM virialization in FIRE is associated with an abrupt change in the properties of star formation and outflows in the central galaxy, and with the formation of a galaxy disc. 
David Eichler


On the origin of the ultra-high energy cosmic-rays נעמי גלובוס
נעמי גלובוס

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.

אופק בירנהולץ
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.
Gravitational Lensing by Galaxy Clusters: Unveiling the Dark Universe אלינור מידז׳ינסקי
אלינור מידז׳ינסקי

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: From Detection to Characterization אביב אופיר
אביב אופיר

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.

יוסי שוורצולד

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.