2006ApJ...646L.111M

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 µm), the average polarization position angles are all found to be consistent with a rotation measure of -(4.4±0.3)x10⁵ rad.m^–2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~(0.2-4)x10^–8 M_☉.yr^–1 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_S. 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.