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          Institute: MPI für Radioastronomie     Collection: Publikationen des MPI für Radioastronomie     Display Documents

ID: 11050.0, MPI für Radioastronomie / Publikationen des MPI für Radioastronomie
The spectrum and variability of circular polarization in Sagittarius A* from 1.4 to 15 GHz
Authors:Bower, G. C.; Falcke, H.; Sault, R. J.; Backer, D. C.
Date of Publication (YYYY-MM-DD):2002-06-01
Title of Journal:Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:2
Start Page:843
End Page:855
Review Status:Peer-review
Audience:Experts Only
Abstract / Description: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 alpha approximate to 0.5 +/- 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 are in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hr was observed during this are, 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 (alpha similar to 5) during this are. We tentatively conclude that the spectrum has two components that match the high- and low-frequency total intensity components.
Free Keywords:galaxies : active; Galaxy : center; polarization; radiation mechanisms : nonthermal; scattering
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Radioastronomie
External Affiliations:Natl Radio Astron Observ, POB O, Socorro, NM 87801 USA; Natl Radio Astron Observ, Socorro, NM 87801 USA; Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA; Univ Calif Berkeley, Radio Astron Lab, Berkeley, CA 94720 USA; Max Planck Inst Radioastron, D-53121 Bonn, Germany; Australia Telescope Natl Facil, Epping, NSW 1710, Australia
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