THE OPTICAL COUNTERPART OF THE ISOLATED NEUTRON-STAR RX J185635-3754

The extreme densities(1) of neutron stars make them an ideal system in which to investigate the equation of state of nuclear matter; accurat e determinations of neutron star masses and radii are crucial for this , Current observations of neutron stars in binary systems yield masses that are generally consistent with theory(2). But measurements of rad ii are more difficult as they require the detection of thermal radiati on from the surface, which in general is masked by emission from non-t hermal processes in radio pulsars(3) and X-ray binary systems(4). Isol ated radio-quiet neutron stars' offer the best opportunity to observe the surface thermal emission, Here we report the detection of the opti cal counterpart of a candidate isolated neutron star, RX J185635-3754 (ref. 6). Our optical flux data, combined with existing extreme ultrav iolet(7) and X-ray(6) observations, show the spectrum to be approximat ely thermal. By adopting the upper bound to the distance of the source , and assuming a plausible model for the spectral energy distribution, we find that the radius of the object cannot exceed 14 km. This resul t is inconsistent with a number of recent equations of states proposed for neutron stars.