Context :

Aims : X-ray emission is an important diagnostic to study magnetic activity in presumably fully convective , very low-mass stars with an effectively neutral photosphere .

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 \times 10 ^ { 26 } erg/s in the 0.2 –2.0 keV band .
The corresponding activity level of log L _ { X } / L _ { bol } \approx - 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 .

Conclusions: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 rather X-ray active , implying the existence of an efficient dynamo mechanisms that is capable of creating magnetic activity and coronal X-ray emission .