RX J1856-3754: Evidence for a Stiff EOS

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.