Polarization Change Due to Fast Winds from Accretion Disks

A fraction of the radiation produced by an accretion disk may be Thomsonscattered by a wind flowing away from the disk. Employing a simpleplane-parallel model of the wind, we calculate the polarization of thescattered radiation and find that its sign depends on the wind velocity,beta=v/c. In the case, 0.12 < beta < 0.78, the polarization is parallel to thedisk normal, i.e., it is orthogonal to the standard Chandrasekhar'spolarization expected from accretion disks. The velocity of anelectron-positron wind is likely to saturate near the equilibrium value beta_*of order 0.5 for which the accelerating radiation pressure is balanced by theCompton drag. Then the change of polarization by the wind is most pronounced.This may help to reconcile the standard accretion disk model with the opticalpolarimetric observations of non-blazar AGNs.Comment: accepted for publication in ApJ Letter