Very Large Array Multiband Monitoring Observations of M31*
Abstract
The Andromeda galaxy (M31) hosts one of the nearest and most quiescent supermassive 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 multifrequency, 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 M31*. For the first time, we detect M31* at 10, 15, and 20 GHz and measure its spectral index, α ≈ -0.45 ± 0.08 (S ν ∠ν α ), 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 M31* at 6 GHz, on timescales of hours to days. Furthermore, a curious decrease of the mean flux density, by ∼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 M31* starting in 2006.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2017
- DOI:
- arXiv:
- arXiv:1707.08317
- Bibcode:
- 2017ApJ...845..140Y
- Keywords:
-
- galaxies: nuclei;
- galaxies: individual: M31;
- radio continuum: galaxies;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 18 pages, 3 figures, 2 tables, accepted for publication in ApJ
