Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357
Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: firstname.lastname@example.org
Accepted: 9 February 2010
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, but 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 min. The quasi-quiescent X-ray luminosity of log LX = 26.2 erg/s and the corresponding activity level of log LX/Lbol = –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, 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 LX = 8 × 1027 erg/s is reached.
Conclusions. 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.
Key words: stars: activity / stars: coronae / stars: individual: SCR 1845-6357 / stars: low-mass / brown dwarfs / X-rays: stars
© ESO, 2010