The Puzzle of The Lyman Continuum Polarization of QSOs

Abstract

Recent spectropolarimetry of QSOs has revealed a surprising rise in polarization in the Lyman continuum of several objects. We discuss several recent attempts to interpret this feature, including the role of Lyman limit absorption in PG 1222+228. We present new theoretical results involving electron scattering in a hot corona or wind above an accretion disk, and polarization resulting from the relativistic returning radiation. These mechanisms can lead to potentially observable polarization at short wavelengths, but neither has quantitative success in fitting the observed Lyman continuum polarization rises. A renewed capability for ultraviolet spectropolarimetry from space is urgently needed to provide further clues to the nature of this phenomenon.