  In a cool neutron star (T < 1.e6 K) endowed with a rather high magnetic field
(B > 1.e13 G), a phase transition may occur in the outermost layers. As a
consequence the neutron star becomes `bare', i.e. no gaseous atmosphere sits on
the top of the crust. The surface of a cooling, bare neutron star does not
necessarily emit a blackbody spectrum because the emissivity is strongly
suppressed at energies below the electron plasma frequency, omega_p. Since
omega_p ~ 1 keV under the conditions typical of the denseelectron gas in the
condensate, the emission from a T ~ 100 eV bare neutron star will be
substantially depressed with respect to that of a perfect Planckian radiator at
most energies. Here we present a detailed analysis of the emission properties
of a bare neutron star. In particular, we derive the surface emissivity for a
Fe composition in a range of magnetic fields and temperatures representative of
cooling isolated neutron stars, like RX J1856.5-3754. We find that the emitted
spectrum is strongly dependent on the electron conductivity in the solid
surface layers. In the cold electron gas approximation (no electron-lattice
interactions), the spectrum turns out to be a featureless depressed blackbody
in the 0.1-2 keV band with a steeper low-energy distribution. When damping
effects due to collisions between electrons and the ion lattice (mainly due to
electron-phonon interactions) are accounted for, the spectrum is more depressed
depending on the magnetic field strength. Details of the emitted spectrum are
found, however, to be strongly dependent of the assumed treatment of the
transition from the external vacuum to the metallic surface. The implications
of out results to RX J1856.5-3754 and other isolated neutron stars are
discussed.
