A pulsar-atmosphere model for PSR 0656+14

We present a pulsar-atmosphere (PA) model for modulated thermal X-ray emission from cooling magnetized neutron stars. The model synthesizes the spectral properties of detailed stellar atmosphere calculations with the non-uniform surface properties anticipated for isolated, aging radio pulsars, and general relativistic effects on photon trajectories. We analyze the archival Chandra observations of the middle-aged radio pulsar PSR 0656+14 with the PA model and find it is an excellent representation of the phase-averaged X-ray spectrum for a broad range of polar effective temperature T_p and column density N_H ~ 10^{20} cm^{-2}. The spectral fits favor a sub-solar neutron star mass M < 1.0 M_{\sun}, a large radius R=15-16 km for a source distance d ~ 190 pc, while retaining consistency with theoretical neutron star equations-of-state. The modulated spectrum constrains the angular displacement of the pulsar spin axis to 30 +/- 2 \arcdeg with respect to the line of sight.