AAS 203rd Meeting, January 2004
Session 63 AGN
Oral, Tuesday, January 6, 2004, 10:00-11:30am, Regency VII

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[63.05] Circular Polarization of AGNs at Centimeter Wavelengths

H.D. Aller, M.F. Aller, R.M. Plotkin, P.A. Hughes (University of Michigan), P.E. Hodge (STScI)

Circular polarization (CP) observations provide a powerful, but relatively unexplored, diagnostic of the magnetic field structure, and particle content, of AGN jets, and a probe of the connection between the parsec-scale jet outflow and the central black hole/accretion disk. We present preliminary results at 4.8 and 8.0 GHz obtained with the University of Michigan 26-meter paraboloid, from a program designed to identify objects with detectable CP and to search for variability (in time and/or frequency) in this emission. The reported results include a reanalysis of data obtained during 1978-1983, and new observations commencing in 2000. To date we have detected CP in 17 objects at one or both frequencies; the strength of the emission is typically on the order of only a few tenths of a percent, and, in the majority of the sources, it is an order of magnitude lower than the fractional linear polarization. In sources followed for a few years or more, CP variations are found to occur typically on timescales of months. The behavior of the CP appears consistent with the presence of stochastic variations produced by mode conversion in transient, opaque, emitting regions in the sources; this interpretation is consistent with CP structure revealed in 15 GHz VLBA maps (Homan and Wardle, AJ, 118, 1942, 1999). Sign reversals in CP have been identified in several strong sources, but we cannot yet rule out the presence of an underlying unidirectional field or helical pattern which might originate in a rotating accretion disk or rotating black hole powering the relativistic flow.

This work is supported in part by NSF grant AST-0307629. The University of Michigan Radio Astronomy Observatory is supported by funds from the Department of Astronomy.

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Bulletin of the American Astronomical Society, 35#5
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