Computer Science
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002hrxs.confe..51z&link_type=abstract
High Resolution X-ray Spectroscopy with XMM-Newton and Chandra, Proceedings of the international workshop held at the Mullard Sp
Computer Science
Scientific paper
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 <~ 10^6 K) endowed with a rather high magnetic field (B >~ 10^13 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.
Drake Jeremy J.
Turolla Roberto
Zane Silvia
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