An X‐Ray Flare Detected on the M8 Dwarf VB 10

We have detected an X-ray flare on the very low mass star VB 10 (GL 752 B; M8V) using the ROSAT High Resolution Imager. VB 10 is the latest type (lowestmass) main sequence star known to exhibit coronal activity. X-rays weredetected from the star during a single 1.1-ksec segment of an observation whichlasted 22 ksec in total. The energy released by this flare is on the order of10^27 ergs/sec. This is at least two orders of magnitude greater than thequiescent X-ray luminosity of VB 10, which has yet to be measured. This X-rayflare is very similar in nature to the far ultraviolet flare which was observedby Linsky et al. (1995) using the Goddard High Resolution Spectrograph onboardHubble Space Telescope. We discuss reasons for the extreme difference betweenthe flare and quiescent X-ray luminosities, including the possibility that VB10 has no quiescent (10^6 K) coronal plasma at all.Comment: 11 pages, 6 figures, Figs. 2 & 3 are embedded in text; Fig. 1a,b,&c are separate file