Toward a Mass and Radius Determination of the Nearby Isolated Neutron Star RX J185635–3754

We discuss efforts to determine the mass, radius, and surface composition of the nearby compact object RX J185635-3754 from its multiwavelength spectral energy distribution. We compute nonmagnetized model atmospheres and emergent spectra for selected compositions and gravities and discuss efforts to fit existing and new observational data from ROSAT, the Extreme Ultraviolet Explorer, and the Hubble Space Telescope. The spectral energy distribution matches that expected from a heavy-element-dominated atmosphere, but not from a uniform-temperature blackbody. Nonmagnetic light-element atmospheres cannot be simultaneously reconciled with the optical and X-ray data. We extend previous studies, which were limited to one fixed neutron star mass and radius. For uniform-temperature models dominated by heavy elements, the redshift z is constrained to be 0.3 ≲ z ≲ 0.4 and the best-fit mass and radius are M ≈ 0.9 M☉ and R ≈ 6 km (for a 61 pc distance). These values for M and R together are not permitted for any plausible equation of state, including that of a self-bound strange quark star. A simplified two-temperature model allows masses and radii up to about 50% larger, or a factor of 2 in the case of a blackbody. The observed luminosity is consistent with the thermal emission of an isolated neutron star no older than about 106 yr, the age inferred from available proper motion and parallax information.