  Recent observations show that the thermal X-ray spectra of many isolated
neutron stars are featureless and in some cases (e.g., RX J1856.5-3754) well
fit by a blackbody. Such a perfect blackbody spectrum is puzzling since
radiative transport through typical neutron star atmospheres causes noticeable
deviation from blackbody. Previous studies have shown that in a strong magnetic
field, the outermost layer of the neutron star may be in a condensed solid or
liquid form because of the greatly enhanced cohesive energy of the condensed
matter. The critical temperature of condensation increases with the magnetic
field strength, and can be as high as 10^6 K (for Fe surface at B \sim 10^{13}
G or H surface at B \sim a few times 10^{14} G). Thus the thermal radiation can
directly emerge from the degenerate metallic condensed surface, without going
through a gaseous atmosphere. Here we calculate the emission properties
(spectrum and polarization) of the condensed Fe and H surfaces of magnetic
neutron stars in the regimes where such condensation may be possible. For a
smooth condensed surface, the overall emission is reduced from the blackbody by
less than a factor of 2. The spectrum exhibits modest deviation from blackbody
across a wide energy range, and shows mild absorption features associated with
the ion cyclotron frequency and the electron plasma frequency in the condensed
matter. The roughness of the solid condensate (in the Fe case) tends to
decrease the reflectivity of the surface, and make the emission spectrum even
closer to blackbody. We discuss the implications of our results for
observations of dim, isolated neutron stars and magnetars.
