Context :

Aims : 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 .

Methods : We investigate an XMM-Newton observation of SCR 1845-6357 , a nearby , ultracool M 8.5 / T 5.5 dwarf binary .
The binary is unresolved in the XMM detectors , however the X-ray emission is very likely from the M 8.5 dwarf .
We compare its flaring emission to those of similar very low mass stars and additionally present an XMM observation of the M 8 dwarf VB 10 .

Results : 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 \times 10 ^ { 27 } erg/s is reached .

Conclusions : 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 .