Accretion Processes in the Nucleus of M31

Abstract

The hypothesized supermassive black hole in the nucleus of M31 (which we shall hereafter call M31*) has many features in common with Sgr A* at the Galactic Center, yet they differ in several significant and important ways. Though M31* is probably ten times heavier, its radio luminosity at 3.6 cm is only one third that of Sgr A*. At the same time, M31* is apparently thousands of times more luminous in X-rays than its Galactic Center counterpart. Thus, a comparative study of these objects can be valuable in helping us to understand the underlying physical basis for their activity. We show here that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius $r_C$ where the ambient gas is captured by the black hole. For typical conditions in the ISM, the initial temperature ($T[r_C]\sim 10^6-10^7$ K) sits on the unstable branch of the cooling function. Depending on the actual value of $T(r_C)$ and the accretion rate, the plasma settles 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 {\it Chandra} observations of M31* reveal it to be a member of the `cold' black hole family. We discuss several predicted features in the spectrum of M31* that may be testable by future multi-wavelength observations, including the presence of a prominent UV spike (from hydrogen line emission) that would be absent on the hot branch.