The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz
Bower G.C.(1,2), Falcke H.(3), Sault R.(4) Backer D.C.(2)
(1) National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville, Socorro, NM 87801(2) Astronomy Department & Radio Astronomy Laboratory, University of California, Berkeley, CA 94720
(3) Max-Planck-Institut für Radioastronomie, Auf den Hüugel 69, D-53121 Bonn, Germany
(4) Australia Telescope National Facility, P.O. Box 76, Epping, NSW, 1710, Australia
The Astrophysical Journal, Vol. 571, p. 843-855 (2002)
Abstract:
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 alpha ~ 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 of 1 hour 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 is strongly inverted (alpha~1.5) during this flare. We tentatively conclude that the spectrum has two components that match the high and low frequency total intensity components.
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Questions: Heino Falcke, hfalcke@mpifr-bonn.mpg.de