The Rotation Measure and 3.5 Millimeter Polarization of Sagittarius A*

We report the detection of variable linear polarization from Sgr A* at awavelength of 3.5mm, the longest wavelength yet at which a detection has beenmade. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 degwith rms scatters of 0.4% and 9 deg over the five epochs. We also detectpolarization variability on a timescale of days. Combined with previousdetections over the range 150-400GHz (750-2000 microns), the averagepolarization position angles are all found to be consistent with a rotationmeasure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotationoccurs external to the polarized source at all wavelengths. This implies anaccretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profilesexpected of ADAF, jet and CDAF models and assuming that the region at whichelectrons in the accretion flow become relativistic is within 10 R_S. Theinferred accretion rate is inconsistent with ADAF/Bondi accretion. Thestability of the mean polarization position angle between disparatepolarization observations over the frequency range limits fluctuations in theaccretion rate to less than 5%. The flat frequency dependence of the inter-daypolarization position angle variations also makes them difficult to attributeto rotation measure fluctuations, and suggests that both the magnitude andposition angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte