------------------------------------------------------------------------
 muno_ps.tex ApJ, October 2004, in press
From: Michael Muno <mmuno@astro.ucla.edu>
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\documentclass[12pt,preprint]{aastex}

\newcommand{\chandra}{{\it Chandra}}
\newcommand{\sgrastar}{\mbox{Sgr A$^*$}}

\begin{document}
\title{The Spectra and Variability of X-ray Sources in a Deep \chandra\ 
Observation of the Galactic Center}
\author{M. P. Muno,\altaffilmark{1,2}
J. S. Arabadjis,\altaffilmark{3}
W. N. Brandt,\altaffilmark{4} P. S. Broos,\altaffilmark{4}
E. D. Feigelson,\altaffilmark{4} G. P. Garmire,\altaffilmark{4}
  M. R. Morris,\altaffilmark{1} and

\altaffiltext{1}{Department of Physics and Astronomy, University of 
California,
Los Angeles, CA 90095; mmuno@astro.ucla.edu}
\altaffiltext{2}{Hubble Fellow}
\altaffiltext{3}{Center for Space Research,
Massachusetts Institute of Technology, Cambridge, MA 02139}
\altaffiltext{4}{Department of Astronomy and Astrophysics,
%525 Davey Laboratory,
The Pennsylvania State University, University Park, PA 16802}

\begin{abstract}
We examine the X-ray spectra and variability of the sample of X-ray
sources with $L_{\rm X} \approx  10^{31} - 10^{33}$~\ergsec\ identified
within the inner 9\arcmin\ of the Galaxy by Muno et al. (2003). Very 
few of
the sources exhibit intra-day or inter-month variations. We find that
the spectra of the point sources near the
Galactic center are very hard
between 2--8~keV, even after accounting for absorption.
When modeled as power laws the median photon index is $\Gamma = 0.7$,
while when modeled as thermal plasma we can only obtain lower limits
to the temperature of $kT > 8$~keV.
The combined spectra of the point sources is similarly hard, with a 
photon
index of $\Gamma = 0.8$. Strong line emission is observed from 
low-ionization,
He-like, and H-like Fe, both in the average spectra and in the brightest
individual sources. The line ratios of the highly-ionized Fe in the
average spectra
are consistent with emission from a plasma in thermal equilibrium. This
line emission is observed whether average spectra are examined as a 
function
of the count rate from the source, or as a function of the hardness 
ratios of
individual sources. This suggests that the hardness of the spectra may 
in
fact be to due local absorption that partially-covers the X-ray emitting
regions in the Galactic center systems. We suggest that most of these
sources are intermediate polars, which
(1) often exhibit hard spectra with
prominent Fe lines, (2) rarely exhibit either flares on short time 
scales
or changes in their mean X-ray flux on long time scales, and
(3) are the most numerous
hard X-ray sources with comparable luminosities in the
Galaxy.
\end{abstract}
\end{document}