Discovery of X-rays from the old and faint pulsar J1154--6250.

We report on the first X-ray observation of the 0.28 s isolated radio pulsar PSR J1154--6250 obtained with the XMM-Newton observatory in February 2018. A point-like source is firmly detected at a position consistent with that of PSR J1154--6250. The two closest stars are outside the 3$\sigma$ confidence limits of the source position and thus unlikely to be responsible for the observed X-ray emission. The energy spectrum of the source can be fitted equally well either with an absorbed power-law with a steep photon index $\Gamma\approx 3.3$ or with an absorbed blackbody with temperature $kT=0.21\pm 0.04$~keV and emitting radius $R_\mathrm{BB} \approx 80$ m (assuming a distance of 1.36~kpc). The X-ray luminosity of $4.4\times 10^{30}$ erg s$^{-1}$ derived with the power-law fit corresponds to an efficiency of $\eta_X = L^\mathrm{unabs}_X/\dot E = 4.5\times 10^{-3}$, similar to those of other old pulsars. The X-ray properties of PSR J1154--6250 are consistent with an old age and suggest that the spatial coincidence of this pulsar with the OB association Cru OB1 is due to a chance alignment.