Tidal Disruption Events in Active Galactic Nuclei: Thermal Radiation and Jets

Abstract: Over the past decade, dozens of Tidal Disruption Events (TDEs) have been identified, with many more expected from upcoming surveys. A small subset has been observed to launch transient, highly luminous relativistic jets, for which the origin of the requisite magnetic flux has not been identified. A non-negligible fraction of TDEs are expected in Active Galactic Nuclei (AGNs), where a magnetized accretion flow is already present. As shown first by Chan et al. (2019), the collision of a TDE debris stream with this flow can substantially alter both disk and debris dynamics, leading to a possibly Eddington-limited flare. Here, we use the Athena++ radiation magnetohydrodynamic (RMHD) code to simulate the time near peak fallback rate of a TDE in an AGN, studying the debris-disk interaction, the radiation emitted, and the accumulation of magnetic flux near the horizon. In the example simulated, the accretion rate climbs to $\sim 12\,\dot{M}_{ \rm Edd}$, while the escaping solid angle-integrated luminosity approaches $\sim L_{ \rm Edd}$. The photon luminosity is strongly asymmetric, brighter in the hemisphere from which the debris strikes the disk and dimmer in the hemisphere toward which the debris travels. The photon spectrum peaks in the EUV. The polar magnetic flux grows $\sim 33$-fold, enough to raise the Blandford--Znajek luminosity by a factor $\sim 10^3$, reaching a level sufficient to power observed TDE jets. The polar magnetic flux is also asymmetric, likely making any jet similarly asymmetric, but in the opposite sense to the photon flux.