Fragmentation in Collapsar Disks: Migration, Growth, and Emission

We present a parameter survey of fragmentation in collapsar disks,
using a revised version of the Chen and Beloborodov (2007) model that
determines the structure of steady state hyperaccretion disks in a general
relativistic and neutrino cooled framework. We map out the range of disk
conditions leading to gravitational instability alongside an exploration of
the dimensionless cooling time β, which together determine whether fragmentation
is likely to occur. We estimate the initial mass and density
of fragments, finding that they occupy a unique region in the space of
self-gravitating compact objects, with masses Mf ∼ 10−3M⊙ − 10−1M⊙
and densities ρf ∼ 108 − 1011 g cm−3. We then calculate their migration
and mass growth (via Bondi-Hoyle accretion) as they inspiral through the
collapsar disk. During a fragment’s migration to the central black hole,
it can grow its mass up to a range Mf ∼ 10−1M⊙ − 1M⊙. In most cases,
the final fragment mass is larger than the minimum cold stable neutron
star mass but much smaller than any observed neutron star. The fragment
briefly achieves peak accretion rates comparable to (or even larger
than) that of the central engine. We propose that these bound fragments
may give rise to observable astrophysical phenomena, and we approximately
model two of these: (i) gamma ray burst variability produced
by a secondary, fragment-launched jet; (ii) the generation of non-vacuum
gravitational waveforms accompanied by electromagnetic counterparts.