X-ray emission from the M9 dwarf 1RXS J115928.5-524717
Quasi-quiescent coronal activity at the end of the main-sequenceJ. Robrade and J. H. M. M. Schmitt
Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
Received 24 October 2008 / Accepted 19 December 2008
Aims. X-ray emission is an important diagnostic for studying magnetic activity in presumably fully convective, very low-mass stars with virtually neutral photospheres.
Methods. We analyse an XMM-Newton observation of 1RXS J115928.5-524717, an ultracool dwarf with spectral type M9, and compare its X-ray properties to those of other similar very late-type stars.
Results. We clearly detected 1RXS J115928.5-524717 at soft X-ray energies in all EPIC detectors. Only minor variability was present during the observation and we attribute the X-ray emission to quasi-quiescent activity. The coronal plasma is described well by a two-temperature model at solar metallicity with temperatures of 2 MK and 6 MK and an X-ray luminosity of about 1026 erg/s in the 0.2-2.0 keV band. The corresponding activity level of log / points to a moderately active star. Altogether, X-ray activity from very low-mass stars shows similar trends as more massive stars, despite their different interior structure.
Conclusions. The nearby star 1RXS J115928.5-524717 is, after LHS 2065, the second ultracool M9 dwarf that emits X-rays at detectable levels in quasi-quiescence. While faint in absolute numbers, both stars are relatively X-ray active, implying an efficient dynamo mechanism that is capable of creating magnetic activity and coronal X-ray emission.
Key words: stars: activity -- stars: coronae -- stars: individual: 1RXS J115928.5-524717 -- stars: low-mass, brown dwarfs -- X-rays: stars
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