A new X-ray look into four old pulsars

We report on the X-ray properties of four rotation-powered pulsars with characteristic ages in the range 0.3-5 Myr, derived from the analysis of XMM-Newton archival observations. We found convincing evidence of thermal emission only in the phase-averaged spectrum of PSR B0114+58, which is well fitted by a blackbody with temperature kT = 0.17 ± 0.02 keV and emitting radius R = 405⁺¹¹⁰_-90 m, consistent with the size of its polar cap. The three other considered pulsars, PSR B0628-28, PSR B0919+06, and PSR B1133+16, have phase-averaged spectra that can be described well by single power laws with photon index Γ 3. The 3σ upper limits on the bolometric luminosity of a possible thermal component with temperatures in the range 0.05-2 keV are L_bol ≲ 3.2 × 10²⁸ erg s^-1 and L_bol ≲ 2.4 × 10²⁹ erg s^-1, for PSR B0628-28 and PSR B0919+06, respectively. On the other hand, we found possible evidence that the pulsed emission of PSR B0628-28 is thermal. Two absorption lines at 0.22 keV and 0.44 keV are detected in the spectrum of PSR B1133+16. They are best interpreted as proton cyclotron features, implying the presence of multipolar components with a field of a few 10¹³ G at the neutron star polar caps. We discuss our results in the context of high-energy emission models of old rotation-powered pulsars.