Is RXJ1856.5-3754 a strange quark star?

Deep Chandra LETGS observations of the isolated neutron star candidate RXJ1856.5-3754 have demonstrated that, to within the accuracy of the observations, the X-ray spectrum is consistent with a blackbody with a temperature of 7 × 105 K and a radiation radius R∞ ∼ 5 km—much too small for current neutron star equations of state. The small apparent radius, lack of X-ray pulsations down to a level of 3%, and failure to explain the observations in terms of current neutron star models, lead to the suggestion that RXJ1856.5-3754 might be a strange quark star. We discuss some issues associated with this interpretation and look briefly at RX J1856.5-3754 in the context of other have smooth featureless spectra. Both X-ray and optical spectra of some of these objects might be explained by “naked” crusted neutron stars or strange quark stars with thin coronae. RX J1856.5-3754 remains an interesting strange quark star candidate.