------------------------------------------------------------------------
From: Frederick K. Baganoff  fkb@space.mit.edu 
To: gcnews@aoc.nrao.edu
Subject: submit chandra_sgra_01.tex ApJ, submitted, 2001 February 2

% http://space.mit.edu/~fkb/GC/chandra_sgra_star_01/ms.ps.gz
% astro-ph/0102151
\documentclass[preprint]{aastex}
\usepackage{emulateapj5}

\newcommand{\sgrastar}{Sgr~A\mbox{$^*$}}
\newcommand{\msun}{\mbox{$\rm M_\sun$}}
\newcommand{\lsun}{\mbox{$\rm L_\sun$}}
\newcommand{\mdot}{\mbox{$\dot{M}$}}
\newcommand{\ldot}{\mbox{$\dot{L}$}}
\newcommand{\ledd}{\mbox{$L_{\rm E}$}}
\newcommand{\mdotedd}{\mbox{$\dot{M}_{\rm E}$}}
\newcommand{\chandra}{\emph{Chandra}}
\newcommand{\cxo}{\emph{CXO}}
\newcommand{\rosat}{\emph{ROSAT}}
\newcommand{\asca}{\emph{ASCA}}
\newcommand{\sax}{\emph{BeppoSAX}}
\newcommand{\ginga}{\emph{Ginga}}
\newcommand{\einstein}{\emph{Einstein}}
\newcommand{\slx}{\emph{Spacelab-2}}
\newcommand{\granat}{\emph{Granat}}
\newcommand{\xmm}{\emph{XMM-Newton}}
\newcommand{\conx}{\emph{Constellation-X}}
\newcommand{\ariel}{\emph{Ariel~V}}
\newcommand{\hipparcos}{\emph{Hipparcos}}
\newcommand{\tycho}{\emph{Tycho-2}}
\newcommand{\usno}{\emph{USNO-A2.0}}

\slugcomment{Submitted to ApJ; 2001 February 2}
\shorttitle{\chandra\ Imaging of \sgrastar\ and the Galactic Center}
\shortauthors{Baganoff et al.}

\begin{document}

\title{\emph{Chandra} X-ray Spectroscopic Imaging of Sgr~A$^*$ \\ and
the Central Parsec of the Galaxy}

\author{F.~K.~Baganoff,\altaffilmark{1} Y.~Maeda,\altaffilmark{2}
        M.~Morris,\altaffilmark{3} M.~W.~Bautz,\altaffilmark{1}
        W.~N.~Brandt,\altaffilmark{2} W.~Cui,\altaffilmark{1,4}
        J.~P.~Doty,\altaffilmark{1} E.~D.~Feigelson,\altaffilmark{2}
        G.~P.~Garmire,\altaffilmark{2} S.~H.~Pravdo,\altaffilmark{5}
        G.~R.~Ricker,\altaffilmark{1} and L.~K.~Townsley\altaffilmark{2}}

%\email{fkb@space.mit.edu}

\altaffiltext{1}{Center for Space Research, Massachusetts Institute of
  Technology, Cambridge, MA 02139-4307; fkb@space.mit.edu}
\altaffiltext{2}{Department of Astronomy and Astrophysics, Pennsylvania
  State University, University Park, PA 16802-6305}
\altaffiltext{3}{Department of Physics and Astronomy, University of
  California, Los Angeles, CA 90095-1562}
\altaffiltext{4}{Department of Physics, Purdue University, West
Lafayette, IN 47907}
\altaffiltext{5}{Jet Propulsion Laboratory, California Institute of
  Technology, Pasadena, CA 91109}

\begin{abstract}

We present results of our \chandra\ observation with ACIS-I centered
on the position of Sagittarius~A$^{*}$ (\sgrastar), the compact
nonthermal radio source associated with the massive black hole (MBH)
at the dynamical center of the Milky Way Galaxy.  We have obtained the
first high spatial resolution ($\approx 1\arcsec$), hard X-ray
(0.5--7~keV) image of the central 40~pc (17\arcmin) of the Galaxy.

We have discovered an X-ray source, \objectname[]{CXOGC
J174540.0$-$290027}, coincident with the radio position of \sgrastar\
to within 0\farcs35, corresponding to a maximum projected distance of
16~light-days for an assumed distance to the center of the Galaxy of
8.0~kpc.  We received $222\pm17$ ($1\sigma$) net counts from the
source in 40.3~ks.  The source is detected with high significance,
$S/N \simeq 37\sigma$, despite the highly elevated diffuse X-ray
background in the central parsec of the Galaxy.  Due to the low number
of counts, the spectrum is well fit either by an absorbed power-law
model with photon index $\Gamma = 2.7^{+1.3}_{-0.9}$ ($N(E) \propto
E^{-\Gamma}$ photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$) and column density
$N_{\rm H} = (9.8^{+4.4}_{-3.0}) \times 10^{22}$ cm$^{-2}$ (90\%
confidence interval) or by an absorbed optically thin thermal plasma
model with $kT = 1.9^{+0.9}_{-0.5}$~keV and $N_{\rm H} =
(11.5^{+4.4}_{-3.1}) \times 10^{22}$ cm$^{-2}$.  Using the power-law
model, the measured (absorbed) flux in the 2--10~keV band is
$(1.3^{+0.4}_{-0.2}) \times 10^{-13}$ ergs cm$^{-2}$ s$^{-1}$, and the
absorption-corrected luminosity is $(2.4^{+3.0}_{-0.6}) \times
10^{33}$ ergs s$^{-1}$.

The X-ray source coincident with \sgrastar\ is resolved, with an
apparent diameter of $\approx 1\arcsec$.  We report the possible
detection, at the $2.7\sigma$ significance level, of rapid continuum
variability on a timescale of several hours.  We also report the
possible detection of an Fe~K$\alpha$ line at the $\simeq 2\sigma$
level.  The long-term variability of \sgrastar\ is constrained via
comparison with the \rosat/PSPC observation in 1992.  The origin of
the X-ray emission (MBH vs.\ stellar) and the implications of our
observation for the various proposed MBH emission mechanisms are
discussed.  The current observations, while of limited
signal-to-noise, are consistent with the presence of both thermal and
nonthermal emission components in the \sgrastar\ spectrum.

We also briefly discuss the complex structure of the X-ray emission
from the Sgr~A radio complex and along the Galactic plane and present
morphological evidence that \sgrastar\ and Sgr~A West lie within the
hot plasma in the central cavity of Sgr~A East.  Over 150 point
sources are detected in the $17\arcmin \times 17\arcmin$ field of
view.  Our survey of X-ray sources is complete down to a limiting
2--10~keV absorbed flux of $F_{\rm X} \approx 1.7 \times 10^{-14}$
ergs cm$^{-2}$ s$^{-1}$.  For sources at the distance of the Galactic
Center, the corresponding absorption-corrected luminosity is $L_{\rm
X} \approx 2.5 \times 10^{32}$ ergs s$^{-1}$.  The complete
flux-limited sample contains 85 sources.  Finally, we present an
analysis of the integrated emission from the detected point sources
and the diffuse emission within the central 0.4~pc (10\arcsec) of the
Galaxy.

\end{abstract}

\keywords{accretion, accretion disks --- black hole physics ---
galaxies: active --- Galaxy: center --- X-rays: ISM, stars}


\end{document}