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 2018 February. A point-like source is firmly detected at a position consistent with that of PSR J1154–6250. The two closest stars are outside the 3σ 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 Γ ≈ 3.3 or with an absorbed blackbody with temperature kT = 0.21 ± 0.04 keV and emitting radius R BB ≈ 80 m (assuming a distance of 1.36 kpc). The X-ray luminosity of 4.4 × 1030 erg s−1 derived with the power-law fit corresponds to an efficiency of ${\eta }_{X}={L}_{X}^{\mathrm{unabs}}/\dot{E}=4.5\times {10}^{-3}$, 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.