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

We present ROSAT All-Sky Survey and ROSAT pointed observations (PSPC and HR I) of a complete sample of 113 bright soft X-ray AGN selected from the ROSA T Bright Source Catalog. We compare these observations in order to search f or extreme cases of flux and spectral X-ray variability - X-ray transient A GN. Three definite transients and one transient candidate are found. The ot her sources show amplitude variations typically by factors of 2-3 on timesc ales 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 object s show stronger variability than flat X-ray spectrum sources. We also prese nt 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 variab ilities observed in the non-transient sources.