Surface Temperature of a Magnetized Neutron Star and Interpretation of the ROSAT Data. I. Dipolar Fields

Abstract

We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observable X-ray spectra and light curves, taking fully into account gravitational lensing. Only dipolar fields are considered here. General features are studied and compared with the ROSAT data from the pulsars 0833-45 (Vela), 0656+14, 0630+178 (Geminga), and 1055-52. The composite blackbody spectra obtained are not very different from a black- body spectrum at the star's effective temperature: temperature estimates using single temperature models are practically equivalent to composite model results. Gravitational lensing strongly suppresses the light curve pulsations: this may be a new tool to constrain the size of neutron stars. Dipolar surface fields yield many features somewhat similar to the observed ones but are not sufficient for an accurate description. The pulsed fractions obtained in all our models increase with photon energy: the strong decrease observed in Geminga at energies 0.3 - 0.5 keV is definitely a genuine effect of the atmospheric magnetic field on the locally emitted spectrum in contradistinction to the magnetic effects on the surface temperature. A complete model will have to consider complex surface field structures and include effects of the atmospheric magnetic field on the emitted spectrum.