Hard X-ray emission from a Compton scattering corona in large black hole mass tidal disruption events

TL;DR

This paper models the nonthermal X-ray emission in tidal disruption events (TDEs) caused by Compton scattering in a corona, linking spectral states to black hole mass and explaining observed X-ray diversity.

Contribution

It introduces a unified analytical and numerical model for TDE X-ray light curves, connecting spectral states with black hole mass and explaining the observed differences in thermal and nonthermal TDE populations.

Findings

Nonthermal X-ray TDEs originate from larger black hole masses.

TDE spectral states transition at a characteristic black hole mass of ~2×10^7 M_sun.

Observed thermal and nonthermal TDE populations have statistically different black hole mass distributions.

Abstract

We extend the relativistic time-dependent thin-disc TDE model to describe nonthermal ($2-10$ keV) X-ray emission produced by the Compton up-scattering of thermal disc photons by a compact electron corona, developing analytical and numerical models of the evolving nonthermal X-ray light curves. In the simplest cases, these X-ray light curves follow power-law profiles in time. We suggest that TDE discs act in many respects as scaled-up versions of XRB discs, and that such discs should undergo state transitions into harder accretion states. XRB state transitions typically occur when the disc luminosity becomes roughly one percent of its Eddington value. We show that if the same is true for TDE discs then this, in turn, implies that TDEs with nonthermal X-ray spectra should come preferentially from large-mass black holes. The characteristic hard-state transition mass is $M_{\rm HS} \simeq 2\times10^7 M_\odot$. Hence, subpopulations of thermal and nonthermal X-ray TDEs should come from systematically different black hole masses. We demonstrate that the known populations of thermal and nonthermal X-ray TDEs do indeed come from different distributions of black hole masses. The null-hypothesis of identical black hole mass distributions is rejected by a two-sample Anderson-Darling test with a $p$-value $< 0.01$. Finally, we present a model for the X-ray rebrightening of TDEs at late times as they transition into the hard state. These models of evolving TDE light curves are the first to join both thermal and nonthermal X-ray components in a unified scenario.