Accretion Disks and the Lyman Continuum Polarization of QSOs

HST observations of some QSOs show a strong, abrupt increase in polarization at rest wavelength about 750 A, suggestive of a connection with the Lyman edge of hydrogen. Blaes and Agol (1996) have proposed an explanation in terms of stellar atmosphere effects in an accretion disk around a supermassive black hole. We have computed the polarized spectrum of a such a disk, including the effects of the relativistic transfer function. Relativistic effects add an additional blueshift of the polarization rise sufficient that the model cannot explain the observations. A good fit results if the emitted radiation is assumed to have a sharp increase in polarized flux at the Lyman edge in the rest frame of the orbiting gas. Relativistic effects then cause the observed polarization to rise sharply at a wavelength substantially less than 912 A. The blueshift depends on the angular momentum of the black hole and the inclination of the disk. A good fit to PG 1630+377 results from a simple model with a dimensionless angular momentum a = 0.5 and an observer viewing angle cos theta = 0.1. An intermediate value of a might result from coallescing black holes, successive accretion events, or electromagnetic extraction of angular momentum from the hole.