Rx J1856.5-3754: Bare Quark Star or Naked Neutron Star?

Recent Chandra observations have convincingly shown that the soft X-ray emission from the isolated neutron star candidate RX J1856.5-3754 is best represented by a featureless blackbody spectrum, in apparent contrast with the predictions of current neutron star atmospheric models. Moreover, the recently measured star distance (≈ 120−140 pc) implies a radiation radius of at most ∼ 5−6 km, too small for any neutron star equation of state. Proposed explanations for such a small radius include a reduced X-ray emitting region (as a heated polar cap), or the presence of a more compact object, as a bare quark/strange star. However, both interpretations rely on the presumption that the quark star or the cap radiates a pure blackbody spectrum, and no justification for this assumption has been presented yet. Here we discuss an alternative possibility. Cool neutron stars (T ∼ < 10 K) endowed with a rather high magnetic field (B ∼ > 10 G) may suffer a phase transition in the outermost layers. As a consequence the neutron star is left bare of the gaseous atmosphere (“naked”). We computed spectra from naked neutron stars with a surface Fe composition. Depending on B, we found that the emission in the 0.1-2 keV range can be featureless and virtually indistinguishable from a blackbody. Moreover, owing to the reduced surface emissivity, the star only radiates ∼ 30−50% of the blackbody power and this implies that the size of the emitting region is larger than for a perfect planckian emitter for the same luminosity. When applied to RX J1856.5-3754 our model accounts for the observed X-ray properties and solves the paradox of the small radius: we predict an apparent star radius of ∼ 10− 12 km, consistent with equations of state of a neutron star. The optical emission of RX J1856.5-3754 may be explained by the presence a thin gaseous shell on the top of the Fe condensate.