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Issue A&A
Volume 367, Number 2, February IV 2001
Page(s) 470 - 486
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20000429



A&A 367, 470-486 (2001)
DOI: 10.1051/0004-6361:20000429

X-ray variability in a complete sample of Soft X-ray selected AGN

D. Grupe1, H.-C. Thomas2 and K. Beuermann1, 3

1  MPI für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
2  MPI für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
3  Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany

(Received 14 August 2000 / Accepted 24 November 2000)

Abstract
We present ROSAT All-Sky Survey and ROSAT pointed observations (PSPC and HRI) of a complete sample of 113 bright soft X-ray AGN selected from the ROSAT Bright Source Catalog. We compare these observations in order to search for extreme cases of flux and spectral X-ray variability - X-ray transient AGN. Three definite transients and one transient candidate are found. The other sources show amplitude variations typically by factors of 2-3 on timescales of years. We found that the variability strength on timescales of days is a function of the steepness of the X-ray spectrum: steeper X-ray objects show stronger variability than flat X-ray spectrum sources. We also present new HRI measurements of our extreme X-ray transients IC 3599 and WPVS007. We discuss possible models to explain the X-ray transience and the variabilities observed in the non-transient sources.


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

Offprint request: D. Grupe, dgrupe@xray.mpe.mpg.de

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© ESO 2001


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