  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 (~10^13 G). Using different
models for the temperature distribution across the neutron star surface
assuming blackbody emission to fit the optical and X-ray spectrum and we derive
a conservative lower limit of the "apparent" neutron star radius of 16.5 km x
(d/117 pc). This corresponds to the radius for the "true" (de-redshifted)
radius of 14 km for a 1.4 Msun 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.
