X-Ray Emitting Supermassive Nuclei of the Local Group
Fulvio Melia, Siming Liu , Mike Fromerth (The University of Arizona), Marco Fatuzzo (Xavier College)
Abstract
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 conducted over the past few years, 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 latter is a two-body
process, which would produce significant X-ray emission only at much higher
accretion rates. This picture leads to the testable prediction that the
physical conditions within the inner
are variable on a time
scale of order a year. In particular, the accretion rate
appears
to have changed by about
between the sub-mm measurements in 1996 and
1999. Given that the radio and self-Comptonized fluxes are strongly
correlated, upcoming multi-wavelength observations of Sgr A* 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. Thus, a comparative study of these
objects can be valuable in helping us to understand the underlying physical
basis for their activity. 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 (
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
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 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.
CATEGORY: ACTIVE GALAXIES AND QUASARS