Large X-ray flares from B-type stars, HD261902 and HD47777, in NGC2264 observed with CHANDRA

The 100 ksec Chandra ACIS-I archival data of high mass star forming region (SFP,) NGC 2264 were analyzed, and 285 X-ray sources were detected, with their luminosities ranging 5 x 10(28) similar to 10(32) erg s(-1). Among them, two B type stars, HD 261902 (138) and HD 47777 (133), showed clear flare like time variations on a typical time scale of similar to 5 hours, both peak X-ray luminosities reaching similar to 10(32) erg/s. In both cases, the time averaged flare spectra can be described by emission model involving two temperatures. The two stars are thus too X-ray luminous for their intermediate mass range, and the flare-like variation is inconsistent with the general lack of magnetic activities in normal B-type stars. These results suggest that some young B-type stars form magnetically active binaries, and can be regarded as much younger analogues of Algol.