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XMM-Newton observation of the very old pulsar J0108-1431

We report on an X-ray observation of the 166 Myr old radio pulsar J0108-1431 with XMM-Newton. The X-ray spectrum can be described by a power-law model with a relatively steep photon index Gamma~3 or by a combination of thermal and non-thermal components, e.g., a power-law component with fixed photon index Gamma~2 plus a blackbody component with a temperature of kT=0.11 keV. The two-component model appears more reasonable considering different estimates for the hydrogen column density. The non-thermal X-ray efficiency in the single power-law model is eta^PL (1-10 keV) = L^PL (1-10 keV) / Edot ~ 0.003, higher than in most other X-ray detected pulsars. In the case of the combined model, the non-thermal and thermal X-ray efficiencies are even higher, eta^PL (1-10 keV) ~ eta^bb ~ 0.006. We detected X-ray pulsations at the radio period of P=0.808s with significance of 7sigma. The pulse shape in the folded X-ray lightcurve (0.15-2 keV) is asymmetric, with statistically significant contributions from up to 5 leading harmonics. Pulse profiles at two different energy ranges differ slightly: the profile is asymmetric at low energies, 0.15-1 keV, while at higher energies, 1-2 keV, it has a nearly sinusodial shape. The radio pulse peak leads the 0.15-2 keV X-ray pulse peak by delta phi = 0.06 +/- 0.03.