Abstract
We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5 mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1%+/-0.1% at a position angle of 16�+/-2� with rms scatters of 0.4% and 9� over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400 GHz (750-2000 mum), the average polarization position angles are all found to be consistent with a rotation measure of -(4.4+/-0.3)�0<SUP>5</SUP> rad m<SUP>-2</SUP>. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~(0.2-4)�0<SUP>-8</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP> for the accretion density profiles expected of ADAF, jet, and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10R<SUB>S</SUB>. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the interday polarization position angle variations also makes them difficult to attribute to rotation-measure fluctuations and suggests that both the magnitude and position angle variations are intrinsic to the emission.
