X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole

TL;DR

This study confirms M31* as an X-ray and radio source, revealing its variability and spectral properties, and provides detailed measurements of its surrounding hot gas within the Bondi radius.

Contribution

It presents the first detailed measurement of M31*'s X-ray spectrum, variability, and the properties of the hot gas within its Bondi radius, based on multi-epoch Chandra observations.

Findings

M31* shows variability in X-ray and radio emissions.

The X-ray spectrum of M31* is consistent with a power-law with slope 0.9.

The hot gas within the Bondi radius has temperature ~0.3 keV and density 0.1 cm^-3.

Abstract

We confirm our earlier tentative detection of M31* in X-rays and measure its light-curve and spectrum. Observations in 2004-2005 find M31* rather quiescent in the X-ray and radio. However, X-ray observations in 2006-2007 and radio observations in 2002 show M31* to be highly variable at times. A separate variable X-ray source is found near P1, the brighter of the two optical nuclei. The apparent angular Bondi radius of M31* is the largest of any black hole, and large enough to be well resolved with Chandra. The diffuse emission within this Bondi radius is found to have an X-ray temperature ~0.3 keV and density 0.1 cm-3, indistinguishable from the hot gas in the surrounding regions of the bulge given the statistics allowed by the current observations. The X-ray source at the location of M31* is consistent with a point source and a power law spectrum with energy slope 0.9+/-0.2. Our identification of this X-ray source with M31* is based solely on positional coincidence.