AAS 198th Meeting, June 2001
Session 74. QSOs
Display, Thursday, June 7, 2001, 9:20am-4:00pm, Exhibit Hall

## [74.07] X-Ray Emitting Supermassive Nuclei of the Local Group

S. Liu, F. Melia, M. Fromerth (The University of Arizona), M. Fatuzzo (Xavier College)

In this talk, we will focus on a theoretical interpretation of what we have learned with recent Chandra observations of the supermassive black hole at the Galactic center (Sgr A*) and in the nucleus of M31 (M31*). The recent detection of Sgr A* in the X-ray band, together with the radio polarization measurements, offer the best constraints yet for understanding the nature of the emitting gas within several Schwarzschild radii (rS) of this object. It now appears that the sub-mm radiation from this source may be associated with thermal synchrotron emission from an inner Keplerian region within the circularization radius of the accreting plasma. In this talk, we discuss the implied high-energy emission of Sgr A* associated with the orbiting, hot, magnetized gas. For the accretion rate inferred from the fits to the sub-mm data, the dominant contribution to Sgr A*'s X-ray flux is due to self-Comptonization of the radio photons, rather than from bremsstrahlung. The accretion rate \dot M appears to have changed by about five halves between the sub-mm measurements in 1996 and 1999. Given that the radio and self-Comptonized fluxes are strongly correlated, upcoming multi-wavelength observations may provide the direct evidence required to test this picture.

M31* has many features in common with Sgr A*, 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. We will discuss how the accretion model developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius rC where the ambient gas is captured by the black hole. For typical conditions in the ISM, the initial temperature (T[rC]~106-107 K) sits on the unstable branch of the cooling function. Depending on the actual value of T(rC) and the accretion rate, the plasma settles either onto a hot branch (attaining a temperature as high as 1010 K or so at small radii) or a cold branch, in which T drops to ~104 K. Sgr A* is presumably a hot' black hole. We show here that the VLA, UV and Chandra observations of M31* reveal it to be a member of the cold' black hole family. We discuss several predicted features in the spectrum that may be testable by future multi-wavelength observations.

The author(s) of this abstract have provided an email address for comments about the abstract: liusm@physics.arizona.edu