Model atmospheres for cooling neutron stars

Abstract

A number of investigators have computed the surface temperature of a cooling neutron star as a function of time after its birth and of the physics of the high-density interior. Einstein observations of supernova remnants and radio pulsars in the soft X-ray band have confronted this cooling theory with rather low upper limits to the surface temperature and a few possible detections, based on the assumption that the surface emits as a blackbody. The authors examine the effect of various surface compositions on the blackbody assumption, calculating model atmospheres for the physical conditions typical of neutron star surfaces with realistic opacities. It is found that, for hydrogen- or helium-dominated surfaces or neutron stars with very low effective temperature, the soft X-ray flux can be much greater than the blackbody value. If high-Z elements dominate the surface, the number of counts expected is comparable to the blackbody value. In this case, however, it is shown that absorption edges will be prominent in the spectrum.