2018A&A...615A..73R

We report on the X-ray properties of four rotation-powered pulsars with characteristic ages in the range 0.3-5Myr, 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±0.02keV and emitting radius R=405⁺¹¹⁰_–90m, 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 ∼0.05-2keV are L_bol≤3.2x10²⁸erg/s and L_bol≤2.4x10²⁹erg/s, 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 ∼0.22keV and ∼0.44keV 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 10¹³G at the neutron star polar caps. We discuss our results in the context of high-energy emission models of old rotation-powered pulsars.