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Astron. Astrophys. 330, 25-36 (1998)


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New bright soft X-ray selected ROSAT AGN

I. Infrared-to-X-ray spectral energy distributions *

D. Grupe1 , ** , K. Beuermann 1, 2, H.-C. Thomas 3, K. Mannheim 1 and H.H. Fink2 , [FORMULA]

1 Universitäts-Sternwarte, Geismarlandstr. 11, D-37083 Göttingen, Germany
2 MPI für extraterrestrische Physik, Giessenbachstr. 6, D-85748 Garching, Germany
3 MPI für Astrophysik, Karl-Schwarzschildstr. 1, D-85748 Garching, Germany

Received 19 September 1996 / Accepted 9 September 1997

Abstract

We present results of an infrared-to-X-ray study of 76 bright soft X-ray selected Seyfert galaxies discovered in the ROSAT All-Sky Survey. These objects are characterized by steep X-ray spectra in the 0.2-2.0 keV bandpass with power law energy spectral indices in the range of 1.3 to 8 and a lack of internal absorption by neutral hydrogen. Our sample selection based on hardness ratio yields a mean slope of [FORMULA] ([FORMULA]), steeper than in any other known AGN population. At optical wavelengths, the soft AGN have significantly bluer spectra than a comparison sample of AGN with a canonical, harder X-ray spectrum, whereas the slope between 5500 Å  and 1 keV is the same. This is consistent with a more pronounced Big Blue Bump emission component in the soft X-ray selected AGN. The blueness of the optical spectra increases with the softness of the X-ray spectra and with the luminosity, saturating at an approximate [FORMULA] spectrum. Such properties are expected if most of the Big Blue Bump emission originates in a (comptonized) accretion disk and [FORMULA]   is higher than in AGN with a hard X-ray spectrum.

Key words: accretion, accretion disks – galaxies: active – galaxies: nuclei – quasars: general – galaxies: Seyfert

* Based in part on observations at the European Southern Observatory La Silla (Chile) with the 2.2m telescope of the Max-Planck-Society during MPG and ESO time, and the ESO 1.5m telescope
** Present address MPI für extraterestrische Physik, Garching, FRG; guest observer: McDonald Observatory, U of Texas at Austin.
[FORMULA] Deceased December 23, 1996.

Send offprint requests to: grupe@usw052.dnet.gwdg.de

© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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