ROSAT ALL-SKY X-RAY SURVEY OF THE CORE REGION OF THE PLEIADES CLUSTER

We present the ROSAT all-sky survey observations of the core region of the Pleiades cluster. A total of 24 X-ray sources are detected and identified with known Pleiades members; 20 X-ray sources had already previously been detected as X-ray emitters with the Einstein Observatory, 3 objects represent new detections, the status of one object is unclear. We show that the Pleiades when viewed as a statistical ensemble of X-ray sources look identical in both the Einstein and ROSAT observations. However, inspection of the activity levels of individual stars shows that changes of more than one order of magnitude have occurred in some stars over a time scale of ten years; we present a statistical method to properly construct the distribution function for the data set consisting of the measured Einstein IPC and ROSAT PSPC count rate ratios, which contains detections and upper and lower limits simultaneously. We argue that the observed large-scale variations in X-ray flux are in general not due to rotational modulation or flare events, but may be signatures of cyclic activity. We also report the detection of an X-ray super flare on the rapidly rotating Pleiades star HII2034.