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 ofthe nearby compact object RX J185635-3754 from its multi-wavelength spectralenergy distribution. We compute non-magnetized model atmospheres and emergentspectra for selected compositions and gravities, and discuss efforts to fitexisting and new observational data from ROSAT, EUVE and the HST. The spectralenergy distribution matches that expected from a heavy-element dominatedatmosphere, but not from a uniform temperature blackbody. Non-magnetic lightelement atmospheres cannot be simultaneously reconciled with the optical andX-ray data. We extend previous studies, which were limited to one fixed neutronstar mass and radius. For uniform temperature models dominated by heavyelements, the redshift z is constrained to be 0.3 < z < 0.4 and the best-fitmass and radius are M approx 0.9 solar masses and R approx 6 km (for a 61 pcdistance). These values for M and R together are not permitted for anyplausible 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 black body. The observed luminosityis consistent with the thermal emission of an isolated neutron star no olderthan about 1 million years, the age inferred from available proper motion andparallax informationComment: Submitted to ApJ 3/9/01; revised 7/19/0