Tidal Disruption Flares: The Accretion Disk Phase

TL;DR

This paper models the evolution of accretion disks formed after stellar disruption by black holes, analyzing how their properties influence observable light curves and X-ray data, with implications for understanding tidal disruption events.

Contribution

It provides a numerical study of accretion disk evolution in tidal disruption flares, highlighting the impact of physical properties on light curves and correcting temperature estimates from X-ray fits.

Findings

Black body fits overestimate true disk temperature.

Peak luminosity occurs at the start of accretion.

Decay timescale of luminosity is about 3-4 years.

Abstract

The evolution of an accretion disk, formed as a consequence of the disruption of a star by a black hole, is followed by solving numerically the hydrodynamic equations. The present investigation aims to study the dependence of resulting light curves on dynamical and physical properties of such a transient disk during its existence. One of main results derived from our simulations is that black body fits of X-ray data tend to overestimate the true mean disk temperature. The temperature derived from black body fits should be identified with the color X-ray temperature rather than the average value derived from the true temperature distribution along the disk. The time interval between the beginning of the circularization of the bound debris and the beginning of the accretion process by the black hole is determined by the viscous timescale, which fixes also the raising part of the resulting light curve. The luminosity peak coincides with the beginning of matter accretion by the black hole and the late evolution of the light curve depends on the evolution of the debris fallback rate. Peak bolometric luminosities are in the range 10^45-10^46 erg s^-1 whereas peak luminosities in soft X-rays (0.2-2.0 keV) are typically one order of magnitude lower. The timescale derived from our preferred models for the flare luminosity to decay by two orders of magnitude is about 3-4 years. Predicted soft X-ray light curves were fitted to data on galaxies in which a variable X-ray emission, related to tidal events, was detected.