Large X-ray flare from the Herbig Be star MWC 297

Hard X-ray emissions from the Herbig Be star MWC 297 were discovered in three separate observations spanning 5 days in 1994 April 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 approximate to 4.9 x 10(32) ergs s(-1) which is 5 times larger than that of the quiescent phase (the first observation). It then declined with an e-folding time of approximate to 5.6 x 10(4) s to the preflare level in the third observation. The X-ray spectra are explained by absorbed thin thermal plasma models. The temperature of approximate to 2.7 keV in the quiescent phase is significantly higher than those of main-sequence OB stars and similar to those of 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 MWC 297, are attributable to magnetic activity similar to that in low-mass YSOs. Since no theory predicts surface convection zones in massive stars like MWC 297, 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.