On the origin of polarization near the Lyman edge in quasars

Optical/UV radiation from accretion disks in quasars is likely to be partly scattered by a hot plasma enveloping the disk. We investigate whether the scattering may produce the steep rises in polarization observed blueward of the Lyman limit in some quasars. We suggest and assess two models. In the first model, primary disk radiation with a Lyman edge in absorption passes through a static ionized "skin" covering the disk, which has a temperature kT similar to 3 keV and a Thomson optical depth tau(T) similar to 1. Electron scattering in the skin smears out the edge and produces a steep rise in polarization at lambda < 912 Angstrom. In the second model, the scattering occurs in a hot coronal plasma outflowing from the disk with a mildly relativistic velocity. We find that the second model better explains the data. The ability of the models to fit the observed rises in polarization is illustrated with the quasar PG 1630+377.