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%astro-ph/0503124

\documentclass[12pt,preprint]{aastex}

\shorttitle{Intra-Day Variability of Sagittarius A$^*$
at 3 
Millimeters}
\shortauthors{Mauerhan et al.}

\begin{document}
\title{Intra-day Variability of Sagittarius A$^*$ at 3
Millimeters}
\author{Jon C. Mauerhan\altaffilmark{1}, Mark
and Frederick K. Baganoff\altaffilmark{3}}
University of California, Los Angeles, Los Angeles, CA
K\"onigstuhl 17, 69117 Heidelberg, Germany;
walter@mpia.de}
\altaffiltext{3}{Center for Space Research,
Massachusetts Institute of Technology, Cambridge MA
02139; 
fkb@space.mit.edu}

\begin{abstract}
We report observations and analysis of flux monitoring
of Sagittarius 
A$^*$ at 3-mm wavelength using the OVRO millimeter
interferometer over a period of 
eight days (2002 May 23--30). Frequent phase and flux 
referencing (every 5 minutes) with the nearby
calibrator source J1744$-$312 was employed 
to control for instrumental and atmospheric effects.
Time variations are 
sought by computing and subtracting, from each
visibility in the 
database, an average visibility obtained from all the
data acquired in our 
monitoring program having similar $\mathit{uv}$
spacings. This removes the 
confusing effects of baseline-dependent, correlated
flux interference 
caused by the static, thermal emission from the
extended source Sgr A West. 
Few-day variations up to $\sim{20\%}$ and intra-day
variability of 
$\sim{20\%}$ and in some cases up to $\sim{40\%}$ on
few-hour time scales emerge 
from the differenced data on SgrA$^*$. Power spectra
of the residuals 
indicate the presence of hourly variations on all but
two of the eight days. 
Monte Carlo simulation of red-noise light curves
indicates that the hourly variations are well
described by a red-noise power spectrum with
$P(f)\propto{f^{-1}}$. Of particular interest is a
$\sim{2.5}$ hour variation seen prominently 
on two consecutive days. An average power spectrum
from all eight days 
of data reveals noteworthy power on this time scale.
There is some 
indication that few-hour variations are more
pronounced on days when the 
average daily flux is highest. We briefly discuss the
possibility that 
these few-hour variations are due to the dynamical
modulation of accreting 
gas around the central supermassive black hole, as
well as the 
implications for the structure of the SgrA$^*$
photosphere at 3 mm. Finally, 
these data have enabled us to produce a high 
sensitivity 3-mm map of the extended thermal emission
surrounding SgrA$^*$.
\end{abstract}