2001MNRAS.323..577S

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 52pc from published photometry. Owing to the close proximity of the star (13.6arcmin) to the calibration source and RS CVn binary AR Lacertae, long-term X-ray coverage is available in the ROSAT archive (∼50h spanning 6.5yr). Two large flare events occurred early in the mission (1990 June-July), and the end of a third flare was detected in 1996 June. One flare, observed with the Position Sensitive Proportional Counter (PSPC), had a peak luminosity L_X=1.1x10³⁰erg.s^–1, an e-folding rise time of 2.2h and a decay time of 7h. 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 High Resolution Imager (HRI) flare (peak L_X=2.9x10³⁰erg.s^–1) is also studied. The `background' X-ray emission is also variable - evidence for low-level flaring or microflaring. We find that ≥59 per cent of the HRI counts and ≥68 per cent of the PSPC counts are caused by flares. At least 41 per cent of the HRI exposure time and 47 per cent of the PSPC are affected by detectable flare enhancement.