Accretion-Induced Collapse of Neutron Stars in the Disks of Active
  Galactic Nuclei
  The disks of active galactic nuclei (AGNs) have emerged as a rich environment
for the evolution of stars and their compact remnants. The very dense medium
favors rapid accretion, while torques and migration traps enhance binary
formation and mergers. Both long and short gamma-ray bursts (GRBs) are hence
expected. We show that AGN disks constitute an ideal environment for another
interesting phenomenon: the accretion induced collapse (AIC) of neutron stars
(NSs) to black holes (BHs). Rapid accretion in the dense disks can cause NSs to
grow to the point of exceeding the maximum mass allowed by their equation of
state. General relativistic magnetohydrodynamical simulations have shown that
electromagnetic signatures are expected if the NS is surrounded by a mini-disk
prior to collapse, which then rapidly accretes onto the BH, and/or if the NS is
highly magnetized, from reconnection of the magnetosphere during collapse. Here
we compute the rates of AICs and their locations within the disks for both
isolated NSs, and for (initially stable) NSs formed from NS-NS mergers. We find
that the global AIC rates are $\sim 0.07-20$~Gpc$^{-3}$~yr$^{-1}$, and we
discuss their observable prospects and signatures as they emerge from the dense
disk environments.
