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Astron. Astrophys. 333, 827-840 (1998)

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Scattering and absorption in soft X-ray selected AGN: an optical polarization survey

D. Grupe * 1, 2, Beverley J. Wills 1, D. Wills 1 and K. Beuermann 2

1 Department of Astronomy & McDonald Observatory, University of Texas at Austin, RLM 15.308, Austin, TX 78712, USA
2 Universitäts-Sternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany

Received 19 August 1997 / Accepted 22 December 1997


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 ([FORMULA] 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 viewing a 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 ([FORMULA] and H [FORMULA] /H [FORMULA]), [O III]/H [FORMULA], and cold intrinsic X-ray absorption [FORMULA]. 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.

Key words: polarization – galaxies: active – galaxies: nuclei – galaxies: Seyfert

* Current address: MPE, Giessenbachstr., D-85748 Garching, Germany

Send offprint requests to: D. Grupe (dgrupe@xray.mpe.mpg.de)

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998