Interferometric Detection of Linear Polarization from Sagittarius

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We measured the linear polarization of Sagittarius A* to be 7:20:6% at 230 GHz using the BIMA array with a resolution of 3:60:9 arcsec. This con rms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We nd a position angle 139 4 degrees that diers substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 210 . These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM = 4:3 0:1 10 .

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