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.
Is RX J1856.5-3754 a naked neutron star ?
TUROLLA, ROBERTO;
2004
Abstract
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.Pubblicazioni consigliate
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