Scattering and Absorption in Soft X-ray Selected AGN: An Optical Polarization Survey

Abstract

We have surveyed the optical linear polarization of a completely identified sample of 43 bright soft-X-ray-selected ROSAT AGN. Most (40) of these AGN show low polarization (<1%), and no clear optical reddening. This supports the suggestion from rapid X-ray variability, disk-like spectral energy distributions, and lack of cold X-ray absorption, that we are viewinga bare AGN disk. IRAS F12397+3333 and IRAS 13349+2438 show high polarization increasing to the UV -- clear evidence for scattering. As well as steep, soft-X-ray spectra, they show optical reddening and rapid X-ray variability, but almost no cold X-ray absorption -- a combination that suggests dusty ionized gas along the line-of-sight. Brandt et al. suggested and found these `warm absorbers' for IRAS 13349+2438. IRAS F12397+3333 is a new candidate. Combining our data with the optical and X-ray spectra of the high polarization narrow-line Seyfert 1 nuclei (NLSy1s) investigated by Goodrich reveals strong correlations among optical reddening indicators (such as the optical continuum slope and Balmer decrement), [OIII]/H-beta, and cold intrinsic X-ray absorption. Optical reddening underpredicts the cold X-ray absorption, suggesting dusty warm absorbers in all the highly polarized AGN. The existence of these scattering-polarized and reddened NLSy1s suggests an orientation Unified Scheme within the class of NLSy1s, analogous to that linking Seyfert 1 and Seyfert 2 nuclei. For some highly polarized and optically selected AGN we present new analysis of archival X-ray data, and for the highly polarized AGN new optical spectroscopy is presented in an appendix.