Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.0805a&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #08.05; Bulletin of the American Astronomical Society, V
Astronomy and Astrophysics
Astronomy
Scientific paper
Circular polarization (CP; Stokes V) observations provide unique constraints on the magnetic field topology and degree of order in AGN jets, and a potential direct link between the central engine and the jet, but, because of low CP emission levels from AGNs (typically tens of mJy at most) its characteristic temporal evolution has not been widely exploited as a radio jet probe. To determine CP variability properties on time scales of weeks to years in a sample of radio-bright AGNs, in 2001 we resumed a program of CP measurements carried out from 1978-1983 using the Michigan 26-m paraboloid operating at 8.0 and 4.8 GHz; in late 2003 we commenced observations at 14.5 GHz. The program provides spectra in all 4 Stokes parameters at 3 frequencies with data spanning nearly 3 decades. We find examples of 1) changes in polarity on decade-long time scales; 2) multi-month, frequency-dependent, ordered changes in amplitude (and sometimes changes in polarity) which are largest in V/I at the two lowest frequencies where Faraday effects are expected to dominate; 3) frequency-dependent differences in both amplitude and polarity at a single epoch; and 4) temporal association between detectable CP levels and self-absorption in the emitting region. These properties are suggestive of the generation of CP by linear-to-circular mode conversion (Jones and O'Dell, 1977, ApJ, 215, 236) in a self-absorbed source with a turbulent magnetic field: radiative transfer calculations confirm that this mechanism can reproduce the observed levels of fractional linear and circular polarization and polarity changes with time. Published VLBI imaging data show that the emission is primarily from the radio core, and we speculate that the observed variability is associated with shifts in the tau=1 surface within the jet. This research was partially funded by NSF grants (AST-0307629, AST-0607523) and the U. Michigan Astronomy Department.
Aller Hugh D.
Aller Margo F.
Hughes Philip A.
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