The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz

Abstract

We report here multiepoch, multifrequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic center. Data taken from the Very Large Array (VLA) archive indicate that the fractional circular polarization at 4.8 GHz was -0.31%, with an rms scatter of 0.13%, from 1981 to 1998, in spite of a factor of 2 change in the total intensity. The sign remained negative over the entire time range, indicating a stable magnetic field polarity. In the summer of 1999, we obtained 13 epochs of VLA A-configuration observations at 1.4, 4.8, 8.4, and 15 GHz. These observations employ a new technique that produces an error of 0.05% at 1.4, 4.8, and 8.4 GHz and 0.1% at 14.9 GHz. In 1999 May, September, and October, we obtained 11 epochs of Australia Telescope Compact Array (ATCA) observations at 4.8 and 8.5 GHz. In all three data sets, we find no evidence of linear polarization greater than 0.1%, in spite of strong circular polarization detections. Both VLA and ATCA data sets support three conclusions regarding the fractional circular polarization: (1) the average spectrum is inverted, with a spectral index α ≈ 0.5 ± 0.2; (2) the degree of variability is roughly constant on timescales of days to years; and (3) the degree of variability increases with frequency. We also observed that the largest increase in fractional circular polarization was coincident with the brightest flare in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hr was observed during this flare, indicating an upper limit to the intrinsic size during outburst of 70 AU at 15 GHz. The fractional circular polarization at 15 GHz reached -1.1%, and the spectral index was strongly inverted (α ~ 1.5) during this flare. We tentatively conclude that the spectrum has two components that match the high- and low-frequency total intensity components.