THE DETECTION OF PULSED X-RAY-EMISSION FROM A NEARBY RADIO PULSAR

As part of a search for thermal surface radiation from nearby neutron stars, we have carried out a 45,000 s observation of the nearby radio pulsar PSR 1929 + 10 with the ROSA T PSPC. After background subtractio n, a net of 420 +/- 25 photons in the 0.1-2.0 keV band were detected a t the position of the pulsar, corresponding to a luminosity of 1.2 x 1 0(30) ergs s-1 for a source distance of 250 pc, or approximately 3 x 1 0(-4) of the pulsar's spin-down luminosity. We find coherent pulsation s from PSR 1929+10 at the radio period of 0.2265 s. The folded light c urve is well fitted by a sinusoidal oscillation with a pulsed fraction of about 30%. The total spectrum is fitted by a blackbody with a temp erature T(infinity) almost-equal-to 3.2 x 10(6) K; the implied emittin g area has a radius of less than 50 m. The maximum of the X-ray light curve coincides with the radio pulse, suggesting that we are detecting the hot magnetic polar cap of the star. The temperature limit for the remainder of the stellar surface is T(infinity) < 3 x 10(5) K. The sp atial distribution of the photons is consistent with the instrument po int-spread function. We discuss the implications of these results for the temperature distribution over the surface of the star and use this detection to constrain various heating mechanisms for rotation-powere d neutron stars. We also use a simple model of general relativistic li ght bending near the stellar surface in conjunction with the radio emi ssion geometry and the X-ray pulsed fraction to derive a radius for th e neutron star of R = 9 +/- 3 km.