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

We report here multi-epoch, multi-frequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic Center. Data taken from the 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-array 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 May, September and October of 1999 we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8 and 8.5 GHz. In all three of the data sets, we find no evidence for 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: the average spectrum is inverted with a spectral index α ≈ 0.5±0.2; the degree of variability is roughly constant on timescales of days to years; and, 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 National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville, Socorro, NM 87801; [email protected] Astronomy Department & Radio Astronomy Laboratory, University of California, Berkeley, CA 94720; gbower,[email protected] Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D 53121 Bonn Germany; [email protected] Australia Telescope National Facility, P.O. Box 76, Epping, NSW, 1710, Australia ; [email protected]