A magnetically heated slim disc-corona model for tidal disruption events

Abstract

Tidal disruption events (TDEs) can reach super-Eddington luminosities near their peak. While their optical/UV emission may arise from multiple processes, X-rays are more likely to be produced by the innermost accretion flow close to the black hole, analogous to super-Eddington flows in stellar-mass X-ray binaries (XRBs) and active galactic nuclei (AGNs). Super-Eddington AGNs typically show disc-dominated spectra, with weak and steep coronae, while super-Eddington XRBs show a wider variety of spectral shapes. Here we explore disc-corona coupling in TDEs by developing a super-Eddington ($\dot{m}>1$: slim) disc model in which magnetic reconnection transfers some of the accretion power from the disc into a hot Comptonizing corona. We test the model on the well-monitored X-ray TDE AT2021ehb, and can broadly fit the brightest X-ray spectra with a black-hole mass $M_{\rm BH}\sim10^6$--$10^7\,M_\odot$ at $\dot m\sim 1$, but need variable magnetic coupling to the disc to match the large observed fast changes in coronal contribution. AGN at similarly high luminosity do not show such variable coronae, but the transient XRB outbursts do, suggesting that it may be a common feature of rapidly changing, non-steady accretion flows. As the TDE declines to $\dot{m}\sim 0.01$, the source hardens, but never goes below $Γ\sim 2$, unlike both AGN and XRB at these luminosities. Thus the X-ray spectral evolution in this TDE does not resemble either XRB or AGN in detail, likely pointing to the very different, highly variable nature of the accretion flow in a tidal disruption event.