Polarization near the Lyman Edge in Accretion Disk Atmosphere Models of Quasars

We calculate self-consistently the structure, spectrum, and polarization of optically thick atmospheres under the conditions expected in quasar accretion disks. We consider a pure-hydrogen composition and neglect the effects of external illumination. We show that, blueward of the Lyman edge, the polarization rises above the value expected from an optically thick, pure electron-scattering atmosphere. This arises because of the wavelength dependence of the absorption opacity, combined with the fact that the source function in this region of the spectrum has a steep variation with optical depth. We suggest that this effect may help explain the recent Hubble Space Telescope observations of steep rises in polarization blueward of the Lyman edge in several quasars. We predict that the polarization should continue to rise blueward of the wavelength at which the current observations end, that cooler spectra should be more polarized, and that the polarization angle in this region should be parallel to the disk plane.