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

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

Corresponding author: D. Grupe, dgrupe@xray.mpe.mpg.de

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.