GRRMHD Simulations of State Transitions in Non-Jetted Tidal Disruption Events

TL;DR

This study uses GRRMHD simulations to explore late-stage behavior of TDE debris disks, revealing thermal instability, X-ray spectral changes, and correlations with black hole spin, matching observations of AT2021ehb.

Contribution

First GRRMHD simulations of magnetized TDE debris disks during late stages, linking spectral evolution and disk collapse to black hole spin.

Findings

Disk becomes thermally unstable within 17-46 days.

X-ray luminosity declines by nearly two orders of magnitude after collapse.

Spectral properties match observed TDE AT2021ehb.

Abstract

Circularization of the stream material into a debris cloud during tidal disruption events (TDEs) was recently demonstrated in one of the most accurate long duration TDE simulations to-date. The cooling envelope model (CEM) provides a description of the circularized debris cloud and its emission over time well beyond circularization across different disruption parameters. In the CEM, sub-Eddington accretion rates occur early in TDEs and the debris has a shallow density profile of roughly $\rho \propto r^{-1}$, with Eddington accretion only being achieved after several months. To explore the late stages of the CEM, we perform general relativistic radiation magnetohydrodynamics (GRRMHD) simulations of magnetized tori adapted from the near Eddington phase of the CEM for a $1M_\odot$ star disrupted around a $10^7 M_\odot$ black hole (BH). We find that the disk becomes thermally unstable within 17.1-46.5 days depending on the spin of the BH. Thermal spectra show a soft X-ray excess prior to collapse, with a nearly two order of magnitude decline in X-ray luminosity upon disk collapse. Furthermore, the evolution of the blackbody radius and temperature of our models are correlated with the spin of the black hole. The spectral properties and soft X-ray luminosity in our models are similar to the TDE AT2021ehb, which is a non-jetted TDE with late X-rays and a state transition after $\approx 271$ days.