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J. Donley, W.N. Brandt (Penn State), Th. Boller (MPE-Garching)
We present the first results from a survey designed to systematically study the most variable sources in the soft X-ray sky detected by ROSAT. This survey cross-correlates ROSAT pointed observations with both ROSAT All-Sky Survey (RASS) detections and other ROSAT pointed observations. These correlations provide a systematic means to examine large-amplitude X-ray variability on a timescale of years, and thus far we have focused on sources where the ROSAT count rate is observed to vary by a factor of 12 or more. We discuss the range and distribution of the source variability detected as well as the classes of objects that show large-amplitude X-ray variability. Future plans to determine the nature of interesting, unidentified objects in the survey are addressed. We also place the tightest constraints yet on the space density of giant variable extragalactic objects (e.g., active galaxies showing X-ray `outbursts') based on the number and nature of detections made.