Migration Traps as Variability Attractors. II. Nonmonotonic X-Ray Timing Signatures of Embedded Stellar-Mass Black Holes in AGN Disks

Abstract

Stellar-mass black holes (sBHs) embedded in active galactic nucleus (AGN) disks may affect X-ray variability through magnetic reconnection. As sBHs migrate and accumulate near migration traps, they disturb the disk magnetic field and trigger reconnection events that heat the disk and eject plasma into the overlying corona. The injected plasma dissipates kinetic energy and can trigger the release of magnetic energy already stored in the corona. We calculate the resulting coronal X-ray emission and the disk ultraviolet (UV) variability, including the disk thermal response and reprocessing of incident X-rays. We find that embedded sBHs can initially enhance short-timescale X-ray variability, but the variability becomes suppressed as more sBHs accumulate in the disk. At higher mass accretion rates through AGN disks, this suppression appears earlier. As sBH-associated reconnection events become more frequent, their active regions increasingly overlap, reducing the magnetic energy available to each event from the corona. We also find that variations in the disk ultraviolet (UV) emission follow the X-ray variations with a shorter delay than in the model without sBHs. These results suggest that a growing population of embedded sBHs does not necessarily produce stronger short-timescale X-ray variability.