Thermal radiation from magnetic neutron star surfaces

We investigate the thermal emission from magnetic neutron star surfaces in which the cohesive effects of the magnetic field have produced the condensation of the atmosphere and the external layers. This may happen for sufficiently cool ( T <= 10(6)) atmospheres with moderately intense magnetic fields ( about 10(13) G for Fe atmospheres). The thermal emission from an isothermal bare surface of a neutron star shows no remarkable spectral features, but it is significantly depressed at energies below some threshold energy. However, since the thermal conductivity is very different in the normal and parallel directions to the magnetic field lines, the presence of the magnetic field is expected to produce a highly anisotropic temperature distribution, depending on the magnetic field geometry. In this case the observed flux of such an object looks very similar to a BB spectrum, but depressed by a nearly constant factor at all energies. This results in a systematic underestimation of the area of the emitter ( and therefore its size) by a factor 5 - 10 ( 2-3).