Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21344603a&link_type=abstract
American Astronomical Society, AAS Meeting #213, #446.03; Bulletin of the American Astronomical Society, Vol. 41, p.331
Astronomy and Astrophysics
Astronomy
Scientific paper
We present multi-frequency monitoring results at 4.8, 8.0, & 14.5 GHz in all 4 Stokes parameters obtained with the University of Michigan 26-meter radio telescope for a sample of bright AGN. Since we resumed spectral monitoring of Stokes V (circular polarization) in 2001, we have detected well-defined variability in Stokes V--either in amplitude, in polarity, or both--characterized by measurements which trace a clear pattern in the light curve in 3C 84, 3C 273, 3C 279, 3C 345, 2145+067, OV-236 and, additionally,find tentative evidence for variability in 3C 446 and 3C 454.3. The amplitudes of the fractional circular polarization range from the detection limit of about 0.1% to a maximum of 1%. 3C 84, 3C 273, and OV-236 exhibit a preferred negative polarity during this time window; 3C 279, 3C 345, and 2145-067 show complex behavior characterized by sign differences from frequency-to-frequency or a polarity change over time at a given frequency. The detected low levels of fractional linear polarization, typically a few percent (or lower in the case of 3C 84) support a scenario in which the magnetic field in the emitting region is predominantly turbulent. While there is no apparent direct relation between the amplitudes of the fractional circular and linear polarizations, the levels of these emissions are consistent with generation of the circularly polarized emission by a linear-to-circular mode conversion process. Comparison with dual frequency data obtained using the same instrument during 1977-83 identifies differences in the characteristic variability pattern in individual sources, suggesting that multi-decadal time windows are required for adequate sampling of Stokes V. This work was supported by the NSF and funds from the U. of Michigan.
Aller Hugh D.
Aller Margo F.
Hughes Philip A.
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