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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 XMMdetectors, however the X-ray emission is very likely from the M8.5 dwarf. Wecompare its flaring emission to those of similar very low mass stars andadditionally present an XMM observation of the M8 dwarf VB 10. We detectquasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the0.2-2.0 keV band, as well as a strong flare with a count rate increase of afactor of 30 and a duration of only 10 minutes. The quasi-quiescent X-rayluminosity of log L_x = 26.2 erg/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, thatis accompanied by a significant brightening in the near-UV, plasma temperaturesof 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s isreached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays atdetectable levels in quasi-quiescence, implying the existence of a corona. Thehigh activity level, coronal temperatures and the observed large flare point toa rather active star, despite its estimated age of a few Gyr.

Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357