High-energy emission from tidal disruption events in active galactic nuclei

Abstract

Tidal disruption events (TDEs) taking place in active galactic nuclei (AGNs) are different from ordinary TDEs. In these events, the returning tidal debris stream drills through the pre-existing AGN accretion disk near the stream pericenter, destroying the inner disk in the process, and then intersects with the disk a second time at radii ranging from a few to hundreds of times the pericenter distance. The debris dynamics of such TDEs, and hence their appearance, are distinct from ordinary TDEs. Here we explore the observational signatures of this "second impact" of the stream with the disk. Strong shocks form as the dilute stream is stopped by the denser disk. Compton cooling of the shocked material produces hard X-rays, even soft gamma-rays, with most of the energy emitted between ~10 keV and 1 MeV. The luminosity follows the mass-return rate, peaking between ~$10^{42}$ and $10^{44}$ erg/s. The X-ray hardness and the smoothness of the light curve provide possible means for distinguishing the second impact from ordinary AGN flares, which exhibit softer spectra and more irregular light curves.