Fragmentation in collapsar discs: migration, growth, and emission

ABSTRACT We present a parameter survey of fragmentation in collapsar discs, using a revised version of the W.-X. Chen & A. M. Beloborodov model that determines the structure of steady-state hyperaccretion discs in a general relativistic and neutrino cooled framework. We map out the range of disc conditions leading to gravitational instability alongside an exploration of the dimensionless cooling time $\beta$, 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 $m_{\mathrm f,i} \sim 10^{-3} -10^{-1}\, {\rm M}_\odot$ and densities $\rho _{\rm f}\sim 10^8-10^{11}~{\rm g~cm}^{-3}$. We then calculate their migration and mass growth (via Bondi–Hoyle accretion) as they inspiral through the collapsar disc. During a fragment’s migration to the central black hole, it can grow its mass up to a range $m_{\rm f}\sim 10^{-1} - 1 \,{\rm M}_\odot$. 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; and (ii) the generation of non-vacuum gravitational waveforms accompanied by electromagnetic counterparts.