ROSAT-DETECTION OF A GIANT X-RAY FLARE ON LKH-ALPHA-92

We report the detection of a giant X-ray flare on the classical T Tauri star LkH alpha 92 with the ROSAT PSPC. In this flare the PSPC count rate rose by a factor of more than 100 in a time interval of about 2000 sec. Our X-ray observations cover most of the rise phase as well as part of the decay phase of the flare. We model the X-ray spectra obtained at different flare phases to determine temperature and emission measure of the flaring plasma. Combining the thus derived X-ray luminosity with the observed decay time scale of approximate to 7800 sec, we estimate the total energy release of the flare in the ROSAT PSPC band (0.1 - 2.4 keV) to be approximate to 4 x 10(36) erg, which is more than a factor of 10(4) greater than the energy release in the strongest solar flares and still more than a factor of 100 greater than that in typical flares on T Tauri stars.