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From: Farhad Zadeh  zadeh@northwestern.edu 
To: gcnews@aoc.nrao.edu
Subject: submission

\title{The Variability of  Polarized Radiation from Sgr A*}
\author{F. Yusef-Zadeh, M. Wardle, W. D. Cotton, C. O. Heinke and D. A. 
Roberts}
%http://arxiv.org/abs/0708.3492  ApJL

\begin{abstract}
Sgr A* is variable at radio and submillimeter wavelengths on hourly
time scales showing time delays between the peaks of flare emission as
well as linearly polarized emission at millimeter and sub-mm
wavelengths.  To determine the polarization characteristics of this
variable source at radio frequencies, we present VLA observations of
Sgr A* and report the detection of polarized emission at a level of
0.77$\pm0.01$\% and 0.2$\pm0.01$\% at 43 and 22 GHz, respectively.
The change in the time averaged polarization angle between 22 and 43
GHz corresponds to a RM of -2.5$\pm0.6 \times10^3$ rad m$^{-2}$ with
no phase wrapping (or $\sim 5\times10^4$ rad m$^2$ with 2$\pi$ phase
wrap).  We also note a rise and fall time scale of 1.5 -- 2 hours in
the total polarized intensity.  The light curves of the degree of
linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz.
The available polarization data at radio and sub-mm wavelengths
suggest that the rotation measure decreases with decreasing frequency.
This frequency dependence, and observed changes in polarization angle
during flare events, may be caused by the reduction in rotation
measure associated with the expansion of synchrotron-emitting
blobs.
\end{abstract}