Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. Thisconfirms the previously reported detection with the JCMT 14-m antenna. Our highresolution observations demonstrate that the polarization does not arise fromdust but from a synchrotron source associated with Sgr A*. We see no change inthe polarization position angle and only a small change in the polarizationfraction in four observations distributed over 60 days. We find a positionangle 139 +/- 4 degrees that differs substantially from what was found inearlier JCMT observations at the same frequency. Polarized dust emission cannotaccount for this discrepancy leaving variability and observational error as theonly explanations. The BIMA observations alone place an upper limit on themagnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observationswhen combined with the JCMT observations at 150, 375 and 400 GHz suggest RM=-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faradayscreen. Barring a special geometry or a high number of field reversals, this RMrules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement isinconsistent with high accretion rates necessary in standard advectiondominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for lowaccretion rates as a major factor in the overall faintness of Sgr A*.Comment: accepted for publication in ApJ, 18 pages, 4 figure