6 v 1 1 9 A ug 2 00 3 Bare Quark Stars or Naked Neutron Stars ?

In a cool neutron star (T . 10 K) endowed with a rather high magnetic field (B & 10 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 cooling, bare neutron stars not necessary gives off blackbody radiation because of the strong suppression in the emissivity at energies below the electron plasma frequency ωp. Since ωp ≈ 1 keV under the conditions typical of the dense electron 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 lowenergy distribution. When damping effects due to collisions between electrons and the ion lattice (mainly due to electron-phonons interactions) are accounted for, the spectrum is again a depressed blackbody down to a limiting energy, which depends on the magnetic field and density, and then sharply drops to zero. As a consequence the star surface behaves as a perfect reflector at energies below (typically) some hundred eV. The implications of our results to RX J1856.5-3754 and other isolated neutron stars are discussed. Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy; [email protected] Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK; [email protected] Smithsonian Astrophysical Observatory, MS 3, 60 Garden Street, Cambridge, MA 02138, USA; [email protected]