Below the Lyman Edge: Ultraviolet Polarimetry of Quasars

The Lyman edge at 912 Å is an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. A robust prediction of accretion disk models 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 the broad, partial Lyman edges expected to be common according to most theories, and none show the high polarizations expected longward of the Lyman edge. Still, polarization spectra of a small number of QSOs have shown a rising 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 determine the amount of polarization on both sides of the Lyman limit. For this new sample of six objects, polarizations are low and mostly consistent with zero below the Lyman edge. Another important result of the new data is that it strengthens the conclusion that quasars are generally not polarized significantly just longward of the Lyman edge at ~1000 Å. There is no significant statistical wavelength dependence of the polarization longward of the Lyman edge, indicating that simple plane-parallel atmospheres with scattering-dominated opacity are not significant sources of UV flux in quasars.