  Recent Chandra observations have shown that the soft X-ray spectrum from the
isolated neutron star RX J1856.5-3754 is featureless and best fitted by a
blackbody, in contrast with the predictions of current neutron star atmospheric
models. 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 for such a small radius include a reduced X-ray emitting region
(as a 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 high
magnetic field (B > 10^13 G) may suffer a phase transition in the outermost
layers. The neutron star is then left bare of the gaseous atmosphere. We
computed spectra from naked neutron stars with a surface Fe composition.
Depending on B, we found that the 0.1-2 keV emission can be featureless and
indistinguishable from a blackbody. Owing to the reduced surface emissivity,
the star only radiates ~30-50 % of the blackbody power, thus that the size of
the emitting region is larger than for a perfect planckian emitter. When
applied to RX J1856.5-3754 our model accounts for the observed X-ray properties
and solves the paradox of the small radius. The apparent star radius is ~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.
