2010A&A...513A..12R

X-ray emission is an important diagnostics to study magnetic activity in very low mass stars that are presumably fully convective and have an effectively neutral photosphere. 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, but 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 0.2-2.0keV band, as well as a strong flare with a count rate increase of a factor of 30 and a duration of only 10 min. The quasi-quiescent X-ray luminosity of logL_X=26.2erg/s and the corresponding activity level of logL_X/L_bol=-3.8 point to a fairly active star. Coronal temperatures of up to 5MK and frequent minor variability support this picture. During the flare, which 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=8x10²⁷erg/s is reached. The source 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.