ROSAT observations of PSR 0656 + 14 - A pulsating and cooling neutron star

Observations of the X-ray-emitting radio pulsar PSR 0656 + 14 with the Position Sensitive Proportional Counter on board Rosat on March 25, 1991 are reported. Pulsations at the radio period, about 0.385 s, were detected at a pulsed fraction of 14 +/-2 percent. A blackbody description yields a surface temperature of about (9.0 +/-0.4) x 10 exp 5 K, an equivalent column density of about (1.0 +/-0.2) x 10 exp 20/sq cm, and a stellar radius of 10 km under the assumption of a 500 pc source distance. A positive correlation is found to exist 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 are discussed.