ROSAT OBSERVATIONS OF PSR-0656+14 - A PULSATING AND COOLING NEUTRON-STAR

PSR 0656+14, an X-ray-emitting radio pulsar, was observed for approximately 3200 s with the Position Sensitive Proportional Counter (PSPC) on board ROSAT on 1991 March 25. The source background-subtracted counting rate was 1.96+/-0.03 counts s-1. Pulsations at the radio period, approximately 0.385 s, were detected at a pulsed fraction of 14%+/-2% making PSR 0656+14 one of only a handful of known isolated X-ray-emitting pulsars. The spectral distribution of counts can be described by various model fits. A blackbody description yields a surface temperature of approximately (9.0+/-0.4) x 10(5) K, an equivalent column density of approximately (1.0+/-0.2) x 10(20) cm-2, and a stellar radius of 10 km under the assumption of a 500 pc source distance. An alternative description is a blackbody modified by the presence of a He atmosphere with surface temperature approximately (3.0+/-0.5) x 10(5) K, column density approximately (2.0+/-0.4) x 10(20) cm-1, and a source distance less-than-or-equal-to 250 pc. A positive correlation exists between the phase of the pulsed emission and the X-ray hardness, suggesting that the pulsations have their seat in a "hot spot" on the neutron star surface. Implications of the models with regard to the cooling curve of neutron stars is discussed.