Radio Band Linear Polarization as a Probe of the Origin of Gamma-Ray Emission from Blazars

To test the hypothesis that shocks play a role in the production of the gamma-ray flares detected by Fermi and to identify jet conditions during gamma-ray flares, we are intensively monitoring the linear polarization and total flux density of a core group of about two dozen radio-and-gamma-ray-bright AGN with the University of Michigan radio telescope to look for the expected shock signature: an increase in the degree of linear polarization and an ordered swing in electric vector position angle. We have observed increases in fractional linear polarization of order ten degrees and swings in electric vector position angle of order tens of degrees in several sources during gamma-ray flares. We present centimeter-band light curves for OJ287, PKS 1510-089, and OT 081 illustrating this signature and compare the spectral evolution apparent in the light curves with simulations from new radiative transfer calculations allowing for the propagation of shocks in the relativistic jet outflows at any orientation to the flow direction. These simulations are able to reproduce the primary features of the observed light curves.This research was supported by NASA Fermi grants NNX09AU16G and NNX10AP16G, NSF grant AST-0607523, and by the University of Michigan.