ar X iv : a st ro - p h / 99 11 12 0 v 1 8 N ov 1 99 9 Large X - ray Flare from a Herbig Be star , MWC 297

Hard X-ray emissions from a Herbig Be star MWC297 were discovered in three separate observations spanning 5 days in April 1994 with the Advanced Satellite for Cosmology and Astrophysics (ASCA). An X-ray flare was found at the beginning of the second observation with a maximum luminosity of ≈ 4.9 × 10 ergs/s, which is five times larger than that of the quiescent phase (the first observation). It then declined with an e-folding time of ≈ 5.6 × 10 sec to the pre-flare level in the third observation. The X-ray spectra are explained by absorbed thin-thermal plasma models. The temperature in the quiescent phase of ≈ 2.7 keV is significantly higher than that of main-sequence OB stars and similar to low mass young stellar objects (YSOs) and other Herbig Ae/Be stars observed with ASCA. The temperature increased in the flare phase to about 6.7 keV at the flux maximum, then decreased to 3.2 keV in the decay phase. These facts strongly suggest that X-rays from Herbig Ae/Be stars, at least for MWC297, are attributable to magnetic activity similar to low mass YSOs. Since no theory predicts surface convection zone in massive stars like MWC297, our results may require a mechanism other than the conventional stellar dynamo theory. Possible magnetic activity could be either the stellar interior shear or the inherited magnetic field from the parent molecular cloud. Subject headings: Stars : abundances – Stars : Be – Stars : flare – Stars : magnetic fields – Stars : pre-main-sequence – X-rays : stars