An X-ray flare detected on the M8 dwarf VB 10

We have detected an X-ray hare on the very low mass star VB 10 (GL 752 B; M 8V) using the ROSAT High Resolution Imager. VB 10 is the latest type (lowes t mass) main-sequence star known to exhibit coronal activity. X-rays were d etected from the star during a single 1.1 ks segment of an observation that lasted 22 ks in total. The energy released by this flare is on the order o f 10(27) ergs s(-1). This is at least 2 orders of magnitude greater than th e quiescent X-ray luminosity of VB 10, which has yet to be measured. This X -ray flare is very similar in nature to the far-ultraviolet flare that was observed by Linsky et al. using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. We discuss reasons for the extreme differ ence between the flare and quiescent X-ray luminosities, including the poss ibility that VB 10 has no quiescent (10(6) K) coronal plasma at all.