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Methods: We analyse an XMM-Newton observation of 1RXS J115928.5-524717, anultracool dwarf with spectral type M9 and compare its X-ray properties to thoseof other similar very late-type stars. Results: We clearly detected 1RXSJ115928.5-524717 at soft X-ray energies in all EPIC detectors. Only minorvariability was present during the observation and we attribute the X-rayemission to quasi-quiescent activity. The coronal plasma is described well by atwo-temperature model at solar metallicity with temperatures of 2 MK and 6 MKand an X-ray luminosity of about L_x = 1.0 x 10^26 erg/s in the 0.2-2.0 keVband. The corresponding activity level of log L_x/L_bol = -4.1 points to amoderately active star. Altogether, X-ray activity from very low-mass starsshows similar trends to more massive stars, despite their different interiorstructure. 1RXSJ115928.5-524717 is, after LHS 2065, the second ultracool M9dwarf that emits X-rays at detectable levels in quasi-quiescence. While faintin absolute numbers,both stars are rather X-ray active, implying the existenceof an efficient dynamo mechanisms that is capable of creating magnetic activityand coronal X-ray emission.

X-ray emission from the M9 dwarf 1RXS J115928.5-524717 - Quasi-quiescent coronal activity at the end of the main sequence