X-ray Evidence for Supermassive Black Holes at the Centers of Nearby Galaxies

TL;DR

This paper reviews X-ray observations from Chandra and XMM-Newton that provide evidence for supermassive black holes in galaxy centers, highlighting recent discoveries of non-recurrent X-ray flares likely caused by stellar tidal disruptions.

Contribution

It presents new X-ray spectral features from active galaxies and discusses the detection of extreme flares in non-active galaxies as evidence for supermassive black holes.

Findings

Detection of spectral features in AGN nuclei

Observation of giant X-ray flares in non-active galaxies

Tidal disruption events as a plausible explanation

Abstract

We first present a short overview of X-ray probes of the black hole region of active galaxies (AGN) and then concentrate on the X-ray search for supermassive black holes (SMBHs) in optically non-active galaxies. The first part focuses on recent results from the X-ray observatories Chandra and XMM-Newton which detected a wealth of new spectral features which originate in the nuclear region of AGN. In the last few years, giant-amplitude, non-recurrent X-ray flares have been observed from several non-active galaxies. All of them share similar properties, namely: extreme X-ray softness in outburst, huge peak luminosity (up to ~10^{44} erg/s), and the absence of optical signs of Seyfert activity. Tidal disruption of a star by a supermassive black hole is the favored explanation of these unusual events. The second part provides a review of the initial X-ray observations, follow-up studies,and the relevant aspects of tidal disruption models studied in the literature.