2000A&A...356..949S

We present the results of a systematic search for X-ray flares on young stars observed during ROSAT PSPC observations of the Taurus-Auriga-Perseus sky region. All pointed PSPC observations currently available from the ROSAT Public Data Archive with known pre-main sequence T Tauri Stars or young Pleiads or Hyads in the field of view are analyzed. A study of the activity of late-type stars of different ages provides information on the evolution of their coronal activity, which may be linked to their angular momentum. We develop a criterion for the detection of flares based on the shape of the X-ray lightcurve. Applying our detection method to all 104 PSPC pointings from the archive we find 52 flares. Among them 15 are detected on T Tauri Stars, 20 on Pleiads, and 17 on Hyads. Only the 38 events which can definitely be attributed to late-type stars (i.e. stars of spectral type G and later) are considered in the statistical analysis of the properties of flaring stars. We investigate the influence of stellar parameters such as age, rotation and multiplicity on individual flare parameters and flare frequency. From the total exposure time falling to the share of each sample and the duration of the individual flares we compute a flare rate. We take into account that the detection sensitivity for large X-ray flares depends on the S/N and hence on the stellar distance. The values we derive for the flare rates are 0.86±0.16% for T Tauri Stars, 0.67±0.13% for Pleiads and 0.32±0.17% for Hyads. The flare rate of classical T Tauri Stars may be somewhat higher than that of weak-line T Tauri Stars (F_c=1.09±0.39% versus F_w=0.65±0.16%). Hardness ratios are used to track the heating that takes place during stellar flares. Hardness ratios are evaluated for three distinct phases of the flare: the rise, the decay, and the quiescent (pre- and post-flare) stage. In most cases the hardness increases during the flares as compared to the quiescent state. During both quiescence and flare phase TTSs display the largest hardness ratios, and the Hyades stars show the softest spectrum.