Extensive Serendipitous X - ray Coverage of a Flare Star with ROSAT

We report the serendipitous discovery of a flare star observed with the ROSAT X-ray observatory. From optical spectra, which show strong and variable emission lines of the hydrogen Balmer series and neutral helium, we classify this object as a M3.0Ve star, and estimate a distance of 52 pc from published photometry. Due to the star's close proximity (13.6 ′) to the calibration source and RS CVn binary AR Lacertae, long term X-ray coverage is available in the ROSAT archive (∼50 hours spanning 6.5 years). Two large flare events occurred early in the mission (6-7/1990), and the end of a third flare was detected in 6/1996. One flare, observed with the Position Sensitive Proportional Counter (PSPC), had a peak luminosity L X =1.1×10 30 erg s −1 , an e-folding rise time of 2.2 hours and a decay time of 7 hours. This decay time is one of the longest detected on a dMe star, providing evidence for the possibility of additional heating during the decay phase. A large HRI flare (peak L X =2.9×10 30 erg s −1) is also studied. The " background " X-ray emission is also variable-evidence for low-level flaring or microflaring. We find that ≥59% of the HRI counts and ≥68% of the PSPC counts are due to flares. At least 41% of the HRI exposure time and 47% of the PSPC are affected by detectable flare enhancement.