Is RX J1856.5-3754 a 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 featureless and best represented by a blackbody spectrum, in apparent contrast with the predictions of current neutron star atmospheric models. Moreover, the 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 include a reduced X-ray emitting region (a heated polar cap), or the presence of a bare quark/strange star. Here we discuss an alternative possibility. Cool neutron stars (T≲106 K) endowed with a rather high magnetic field (B≳1013 G) may be left bare of the gaseous atmosphere by a phase transition in the outermost layers. Computed spectra from bare neutron stars with a surface Fe composition are featureless and virtually indistinguishable from a blackbody in the 0.1–2 keV range. Moreover, owing to the reduced surface emissivity, the star only radiates ∼30–50% of the blackbody power and this implies a star radius larger than the radiation radius. Our model can potentially account for the observed X-ray properties of RX J1856.5-3754 and predicts a star radius R∞∼10–12 km. 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.