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%astro-ph/0402543
\documentclass[preprint,12pt]{aastex}
\usepackage{natbib}

\begin{document}

\title{The Variability of Sagittarius~A* at Centimeter Wavelengths}

\author{
Robeson M. Herrnstein\altaffilmark{1},
Geoffrey C. Bower\altaffilmark{3}, and

\altaffiltext{1}{Department of Astronomy, Columbia University, Mail Code
5246, 550 West 120th St., New York, NY \\10027,
herrnstein@astro.columbia.edu}
\altaffiltext{2}{Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, \\jzhao@cfa.harvard.edu}
\altaffiltext{3}{601 Campbell Hall, Radio Astronomy Lab, UC Berkeley,
Berkeley, CA 94720, \\gbower@astron.berkeley.edu}
\altaffiltext{4}{National Radio Astronomy Observatory, P.O. Box 0,
Socorro, NM, 87801, mgoss@aoc.nrao.edu }

\begin{abstract}
We present the results of a 3.3-year project to monitor the flux
density of Sagittarius~A* at 2.0, 1.3, and 0.7~cm with the Very Large
Array.  Between 2000.5 and 2003.0, 119 epochs of data were taken with
a mean separation between epochs of eight days.  After 2003.0,
observations were made roughly once per month for a total of nine
additional epochs.  Details of the data calibration process are
discussed, including corrections for opacity and elevation effects as
well as changes in the flux density scales between epochs.  The fully
calibrated light curves for Sgr~A* at all three wavelengths are
presented.  Typical errors in the flux density are 6.1\%, 6.2\%, and
9.2\% at 2.0, 1.3, and 0.7~cm, respectively.  There is preliminary
evidence for a bimodal distribution of flux densities, which may
indicate the existence of two distinct states of accretion onto the
supermassive black hole.  At 1.3 and 0.7~cm, there is a tail in the
distribution towards high flux densities.  Significant variability is
detected at all three wavelengths, with the largest amplitude
variations occurring at 0.7~cm.  The rms deviation of the flux density
of Sgr~A* is 0.13, 0.16, and 0.21~Jy at 2.0, 1.3, and 0.7~cm,
respectively.  During much of this monitoring campaign, Sgr~A*
appeared to be relatively quiescent compared to results from previous
campaigns.  At no point during the monitoring campaign did the flux
density of Sgr A* more than double its mean value.  The mean spectral
index of Sgr~A* is $\alpha=0.20\pm0.01$ (where
$S_\nu\propto\nu^\alpha$), with a standard deviation of 0.14.  The
spectral index appears to depend linearly on the observed flux density
at 0.7~cm with a steeper index observed during outbursts.  This
correlation is consistent with the expectation for outbursts that are
self-absorbed at wavelengths of 0.7~cm or longer and inconsistent with
the effects of simple models for interstellar scintillation.  Much of
the variability of Sgr~A*, including possible time lags between flux
density changes at the different wavelengths, appears to occur on time
scales less than the time resolution of our observations (8 days).
Future observations should focus on the evolution of the flux density
on these time scales.

\end{abstract}

\keywords{Galaxy: center --- radio continuum: galaxies --- accretion ---
black hole physics}

\section{Introduction \label{intro}}

\end{document}


+--------------------------------------------------+
|Robin M. Herrnstein                               |
|       Dept of Astronomy, Columbia University     |
|       Mail Code 5246                             |
|       550 West 120th St.                         |
|       New York, NY 10027                         |
|office:1318 Pupin Hall                            |
|       212-854-6831                               |
|email: herrnstein@astro.columbia.edu              |
|URL:   http://www.astro.columbia.edu/~herrnstein  |
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