The Evolution of X-ray Spectra in Tidal Disruption Events

TL;DR

This paper introduces a new disc-corona model to explain the observed spectral hardening in TDEs, linking the transition radius between slim disc and disc-corona to decreasing accretion rates, and successfully applied to AT 2019azh.

Contribution

A novel disc-corona model that explains X-ray spectral evolution in TDEs by linking spectral hardening to changes in the transition radius with accretion rate.

Findings

The transition radius decreases as the accretion rate drops.

The model successfully explains spectral hardening in AT 2019azh.

Predictions align with observed spectral evolution in TDEs.

Abstract

The study of the evolution of X-ray spectra in tidal disruption events (TDEs) is an important approach for understanding the physical processes occurring near a supermassive black hole. Observations show that the X-ray spectra of TDEs are very soft at the peak after the outburst, followed by a spectral hardening on a timescale of years. Theoretically, TDEs are suggested to undergo super-Eddington accretion at the time around the outburst. In this paper, we constructed a new disc-corona model to explain the observed X-ray spectral hardening in TDEs. In our model, there is a transition radius $r_{\text{tr}}$. For $r< r_{\text{tr}}$, the accretion flow exists in the form of a slim disc, the emission of which is dominated by soft X-rays. While for $r>r_{\text{tr}}$, the accretion flow exists in the form of a traditionally sandwiched disc-corona, in which a harder X-ray spectrum is produced. Our calculations show that $r_{\text{tr}}$ decreases with decreasing mass accretion rate $\dot {M}$, which intrinsically can predict the hardening of the X-ray spectra since the relative contribution of the outer disc-corona to the inner slim disc to the X-ray spectrum increases with decreasing $\dot {M}$. Our model has been applied to explain the observed X-ray spectral hardening in TDE candidate AT 2019azh, in which $\dot {M}$ is assumed to decrease proportionally to $t^{-5/3}$. Potential applications of the model in explaining the X-ray spectral evolution in upcoming rich TDE observations are also expected.