1995A&A...304L..13P

We present ROSAT X-ray observations as well as optical photometry and spectroscopy of the young stellar object P1724 in Orion. During a ROSAT 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 exponential decay with a very long decay time of about 9 hours. P1724 is not a clear proper motion member to the Orion nebula association and, hence, its distance is not well established, but we present new spectroscopic and photometric evidence indicating that P1724 is a weak-line TTS associated with the Orion star forming region. For a distance of ∼500pc, the total flare energy radiated in the ROSAT X-ray band (0.1-2.4keV) is more than ∼5x10³⁷erg. 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 ever observed on a star.