Very Large Array multi-band monitoring observations of M 31*

TL;DR

This study presents multi-frequency radio observations of M 31* revealing its spectral properties, variability, and potential jet features, providing new insights into the low-luminosity accretion processes of the nearby supermassive black hole.

Contribution

First detection of M 31* at 10, 15, and 20 GHz with spectral analysis, and evidence of flux variability and jet-like features in a comprehensive multi-epoch, multi-frequency radio campaign.

Findings

M 31* has a steep spectral index of about -0.45 between 5-20 GHz.

Detected flux variability of a few tens of percent at 6 GHz over hours to days.

Observed a 50% decrease in mean flux density between 2002-2005 and 2011-2012 observations.

Abstract

The Andromeda galaxy (M\,31) hosts one of the nearest and most quiescent super-massive black holes, which provides a rare, but promising opportunity for studying the physics of black hole accretion at the lowest state. We have conducted a multi-frequency, multi-epoch observing campaign, using the Karl G. Jansky Very Large Array (VLA) in its extended configurations in 2011-2012, to advance our knowledge of the still poorly known radio properties of M\,31*. For the first time, we detect M\,31* at 10, 15 and 20 GHz, and measure its spectral index, $\alpha \approx -0.45\pm0.08$ (S$_{\nu}$ $\varpropto$ $\nu^{\alpha}$), over the frequency range of 5-20 GHz. The relatively steep spectrum suggests that the observed radio flux is dominated by the optically-thin part of a putative jet, which is located at no more than a few thousand Schwarzschild radii from the black hole. On the other hand, our sensitive radio images show little evidence for an extended component, perhaps except for several parsec-scale "plumes", the nature of which remains unclear. Our data also reveal significant (a few tens of percent) flux variation of M\,31* at 6 GHz, on timescales of hours to days. Furthermore, a curious decrease of the mean flux density, by $\sim$50\%, is found between VLA observations taken during 2002-2005 and our new observations, which might be associated with a substantial increase in the mean X-ray flux of M\,31* starting in 2006.