  We have examined the soft X-ray plus optical/UV spectrum of the nearby
isolated neutron star RX J1856-3754, comparing with detailed models of a
thermally emitting surface. Like previous investigators, we find the spectrum
is best fit by a two-temperature blackbody model. In addition, our simulations
constrain the allowed viewing geometry from the observed pulse fraction upper
limits. These simulations show that RX J1856-3754 is very likely to be a normal
young pulsar, with the non-thermal radio beam missing Earth's line of sight.
The SED limits on the model parameter space put a strong constraint on the
star's M/R. At the measured parallax distance, the allowed range for
Mstar=1.5Msun is Rstar=13.7+/-0.6km. Under this interpretation, the EOS is
relatively stiff near nuclear density and the `Quark Star' EOS posited in some
previous studies is strongly excluded. The data also constrain the surface T
distribution over the polar cap.
