We investigate an XMM-Newton observation of SCR 1845-6357, a nearby,
ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM
detectors, however the X-ray emission is very likely from the M8.5 dwarf. We
compare its flaring emission to those of similar very low mass stars and
additionally present an XMM observation of the M8 dwarf VB 10. We detect
quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the
0.2-2.0 keV band, as well as a strong flare with a count rate increase of a
factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray
luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log
L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5
MK and frequent minor variability support this picture. During the flare, that
is accompanied by a significant brightening in the near-UV, plasma temperatures
of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is
reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at
detectable levels in quasi-quiescence, implying the existence of a corona. The
high activity level, coronal temperatures and the observed large flare point to
a rather active star, despite its estimated age of a few Gyr.Comment: Accepted by A&A, 6 pages, 5 figure
