Accretion Processes in the Nucleus of M31

The hypothesized supermassive black hole in the nucleus of M31 (which weshall hereafter call M31*) has many features in common with Sgr A* at theGalactic Center, yet they differ in several significant and important ways.Though M31* is probably ten times heavier, its radio luminosity at 3.6 cm isonly one third that of Sgr A*. At the same time, M31* is apparently thousandsof times more luminous in X-rays than its Galactic Center counterpart. Thus, acomparative study of these objects can be valuable in helping us to understandthe underlying physical basis for their activity. We show here that theaccretion model being developed for Sgr A* comprises two branches of solutions,distinguished by the relative importance of cooling compared to compressionalheating at the radius $r_C$ where the ambient gas is captured by the blackhole. For typical conditions in the ISM, the initial temperature ($T[r_C]\sim10^6-10^7$ K) sits on the unstable branch of the cooling function. Depending on the actual value of $T(r_C)$ and the accretion rate, the plasmasettles either onto a hot branch (attaining a temperature as high as $10^{10}K$ or so at small radii) or a cold branch, in which $T$ drops to $\sim 10^4$ K.Sgr A* is presumably a `hot' black hole. We show here that the VLA, UV and {\itChandra} observations of M31* reveal it to be a member of the `cold' black holefamily. We discuss several predicted features in the spectrum of M31* that maybe testable by future multi-wavelength observations, including the presence ofa prominent UV spike (from hydrogen line emission) that would be absent on thehot branch.Comment: 14 pages, 2 figure