A systematic study of X-ray variability in the ROSAT all-sky survey

We present a systematic search for variability among the ROSAT All-Sky Survey ( RASS) X-ray sources. We generated lightcurves for about 30 000 X-ray point sources detected sufficiently high above background. For our variability study different search algorithms were developed in order to recognize flares, periods and trends, respectively. The variable X-ray sources were optically identified with counterparts in the SIMBAD, the USNO-A2.0 and NED data bases, but a significant part of the X-ray sources remains without cataloged optical counterparts. Out of the 1207 sources classified as variable 767 (63.5%) were identified with stars, 118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified with other sources and 312 (25.8%) could not uniquely be identified with entries in optical catalogs. We give a statistical analysis of the variable X-ray population and present some outstanding examples of X-ray variability detected in the ROSAT all-sky survey. Most prominent among these sources are white dwarfs, apparently single, yet nevertheless showing periodic variability. Many flares from hitherto unrecognised flare stars have been detected as well as long term variability in the BL Lac 1E1757.7+7034.