The puzzles of RX J1856.5-3754:: neutron star or quark star?

We discuss recent Chandra and XMM-Newton observations of the bright isolated neutron star RX J1856.5-3754 and suggest that the absence of any line features is due to effects of a high magnetic field strength (similar to 10(13) G). Using different models for the temperature distribution across the neutron star surface and assuming blackbody emission to fit the optical and X-ray spectrum, we derive a conservative lower limit of the apparent neutron star radius of 16.5 km x (d/117pc). This corresponds to the radius for the true (de-redshifted) radius of 14 km for a 1.4 M-circle dot neutron star, indicating a stiff equation of state at high densities. A comparison of the result with mass-radius diagrams shows that quark stars and neutron stars with quark matter cores can be ruled out with high confidence.