A GIANT X-RAY FLARE ON THE YOUNG STAR P1724

We present ROSAT X-ray observations as well as optical photometry and spectroscopy of the young stellar object P1724 in Orion. During a ROSA T observation in September 1992 P1724 showed a giant flare, where the X-ray count rate rose by more than a factor of 26 and then showed an e xponential decay with a very long decay time of about 9 hours. P1724 i s not a clear proper motion member to the Orion nebula association and , hence, its distance is not well established, but we present new spec troscopic and photometric evidence indicating that P1724 is a weak-lin e TTS associated with the Orion star forming region. For a distance of approximate to 500 pc, the total flare energy radiated in the ROSAT X -ray band (0.1 - 2.4 keV) is more than approximate to 5 X 10(37) erg. This is an order of magnitude above the highest energies of flares on young stars found so far, making this event the largest X-ray flare ev er observed on a star.