X-ray properties of TDEs

TL;DR

This paper reviews the X-ray observational properties of tidal disruption events (TDEs), highlighting their spectral evolution, accretion phases, and implications for black hole physics and intermediate mass black holes.

Contribution

It provides a comprehensive overview of X-ray TDE observations, their spectral and temporal characteristics, and discusses their significance in understanding black hole accretion and black hole demographics.

Findings

X-ray TDEs exhibit high peak luminosities and thermal spectra consistent with stellar debris accretion.

A typical evolution involves near-Eddington accretion, outflows, and a transition to a sub-Eddington phase with a declining flux.

Evidence suggests TDEs can reveal the presence of intermediate mass black holes.

Abstract

Observational astronomy of tidal disruption events (TDEs) began with the detection of X-ray flares from quiescent galaxies during the ROSAT all-sky survey of 1990-1991. The flares complied with theoretical expectations, having high peak luminosities ($L_{\rm x}$ up to $\ge4\times 10^{44}$ erg/s), a thermal spectrum with $kT\sim$few$\times10^5$ K, and a decline on timescales of months to years, consistent with a diminishing return of stellar debris to a black hole of mass $10^{6-8}$ solar masses. These measurements gave solid proof that the nuclei of quiescent galaxies are habitually populated by a super-massive black hole. Beginning in 2000, XMM-Newton, Chandra and Swift have discovered further TDEs which have been monitored closely at multiple wavelengths. A general picture has emerged of, initially near-Eddington accretion, powering outflows of highly-ionised material, giving way to a calmer sub-Eddington phase, where the flux decays monotonically, and finally a low accretion rate phase with a harder X-ray spectrum indicative of the formation of a disk corona. There are exceptions to this rule though which at the moment are not well understood. A few bright X-ray TDEs have been discovered in optical surveys but in general X-ray TDEs show little excess emission in the optical band, at least at times coincident with the X-ray flare. X-ray TDEs are powerful new probes of accretion physics down to the last stable orbit, revealing the conditions necessary for launching jets and winds. Finally we see that evidence is mounting for nuclear and non-nuclear intermediate mass black holes based on TDE flares which are relatively hot and/or fast.