  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 L_x = 1.0 x 10^26 erg/s in the 0.2-2.0 keV
band. The corresponding activity level of log L_x/L_bol = -4.1 points to a
moderately active star. Altogether, X-ray activity from very low-mass stars
shows similar trends to more massive stars, despite their different interior
structure. 1RXSJ115928.5-524717 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 rather X-ray active, implying the existence
of an efficient dynamo mechanisms that is capable of creating magnetic activity
and coronal X-ray emission.
