Below the Lyman edge: Ultraviolet polarimetry of quasars

The Lyman edge at 912 Angstrom is an important diagnostic region for studyi ng quasi-stellar objects (QSOs). In particular, it reveals a great deal abo ut the physical conditions within the atmospheres of accretion disks, a ubi quitous component of QSO theories. A robust prediction of accretion disk mo dels is a significant polarization due to electron scattering just longward (in wavelength) of the Lyman edge, because of the wavelength dependence of the hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features; few QSOs show t he broad, partial Lyman edges expected to be common according to most theor ies, and none show the high polarizations expected longward of the Lyman ed ge. Still, polarization spectra of a small number of QSOs have shown a risi ng polarization (up to 20%) at wavelengths shortward of the Lyman edge. We have now doubled our sample of intermediate-redshift QSOs observed with the Hubble Space Telescope Faint Object Spectrograph spectropolarimeter to det ermine the amount of polarization on both sides of the Lyman limit. For thi s new sample of six objects, polarizations are low and mostly consistent wi th zero below the Lyman edge. Another important result of the new data is t hat it strengthens the conclusion that quasars are generally not polarized significantly just longward of the Lyman edge at similar to 1000 Angstrom. There is no significant statistical wavelength dependence of the polarizati on longward of the Lyman edge, indicating that simple plane-parallel atmosp heres with scattering-dominated opacity are not significant sources of UV f lux in quasars.