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

摘要

Aims. X-ray emission is an important diagnostics to studymagnetic activity in very low mass stars that are presumably fullyconvective and have an effectively neutral photosphere. Methods. We investigate an XMM-Newton observation ofSCR1845-6357, a nearby, ultracoolM8.5/T5.5 dwarf binary. The binary isunresolved in the XMM detectors, but the X-ray emission is very likelyfrom the M8.5 dwarf. We compare its flaring emission to those ofsimilar very low mass stars and additionally present an XMM observationof the M8 dwarf VB10. Results. We detect quasi-quiescent X-ray emission fromSCR1845-6357 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 30and a duration of only 10min. The quasi-quiescent X-rayluminosity of erg/s and the corresponding activity level of /point to a fairly active star. Coronal temperatures of up to 5MKand frequent minor variability support this picture. During the flare,which is accompanied by a significant brightening in the near-UV,plasma temperatures of 25-30MK are observed and an X-rayluminosity of erg/s is reached. Conclusions. The source SCR1845-6357 is a nearby, very lowmass star that emits X-rays at detectable levels in quasi-quiescence,implying the existence of a corona. The high activity level, coronaltemperatures and the observed large flare point to a rather activestar, despite its estimated age of a few Gyr. Key words: stars: activity - stars: coronae - stars: individual: SCR1845-6357 - stars: low-mass - brown dwarfs - X-rays: stars