Tidal Disruption Events in Active Galactic Nuclei

Abstract

A fraction of tidal disruption events (TDEs) occur in active galactic nuclei (AGNs) whose black holes possess accretion disks; these TDEs can be confused with common AGN flares. The disruption itself is unaffected by the disk, but the evolution of the bound debris stream is modified by its collision with the disk when it returns to pericenter. The outcome of the collision is largely determined by the ratio of the stream mass current to the azimuthal mass current of the disk rotating underneath the stream footprint, which in turn depends on the mass and luminosity of the AGN. To characterize TDEs in AGNs, we simulated a suite of stream-disk collisions with various mass current ratios. The collision excites shocks in the disk, leading to inflow and energy dissipation orders of magnitude above Eddington; however, much of the radiation is trapped in the inflow and advected into the black hole, so the actual bolometric luminosity may be closer to Eddington. The emergent spectrum may not be thermal, TDE-like, or AGN-like. The rapid inflow causes the disk interior to the impact point to be depleted within a fraction of the mass return time. If the stream is heavy enough to penetrate the disk, part of the outgoing material eventually hits the disk again, dissipating its kinetic energy in the second collision; another part becomes unbound, emitting synchrotron radiation as it shocks with surrounding gas.

The Effect of Star–Disk Interactions on Highly Eccentric Stellar Orbits in Active Galactic Nuclei: A Disk Loss Cone and Implications for Stellar Tidal Disruption Events

“…This finding does not explain the Tadhunter et al (2017) result of an elevated tidal disruption event rate in AGN, nor does it rule out the coexistence of tidal disruption events with preexisting accretion flows (e.g. Chan et al 2019).…”

“…One such potential event has been observed by Tadhunter et al (2017), who additionally claim that this event also represents a highly elevated occurrence rate of tidal disruptions within AGN. Theoretical models of the hydrodynamic interaction between a tidal disruption debris stream and a preexisting accretion disk have been recently presented by Chan et al (2019).…”

AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy

“…Hung et al 2017). However, recent simulations of TDEs occurring in AGN have shown that the spectrum of the resulting transient may not look like a standard TDE and it is even uncertain in which energies the radiation emerges from the event (Chan et al 2019). Furthermore, substantial reprocessing of the TDE emission may have occurred within the dense BLR clouds.…”

Simulations of Tidal Disruption Events

“…Recently, Chan et al (2019) have simulated the interaction of the debris stream with a pre-existing accretion disc, when the disruption occurs in an AGN. They have used the grid-based code Athena++ and find that the stream can excite significant inflow in the disc (strongly super-Eddington), while the emergent radiation is expected to be close to the Eddington value.…”