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J.A. Pons, F.W. Walter, J.M. Lattimer, M. Prakash (SUNY at Stony Brook)
We present an investigation of the mass (M) and radius (R) determination of the isolated Neutron Star RX J185635-3754. We compute non-magnetized model atmospheres and emergent spectra for different compositions and gravities, and discuss model fits to ROSAT, EUVE and HST/optical data. The spectral energy distribution is consistent with a heavy element (Iron, Silicon) atmosphere, while light element atmospheres seem difficult to reconcile with the data, even with the presence of a strong magnetic field. Our fits allow us to determine the effective temperature and the M/R and R/D ratios. These ratios, together with recent parallax measurements which give a distance to the object of D\approx 60 pc, result in the first direct measurement of the mass and radius of a Neutron Star. The implications on the equation of state of dense matter and the possible presence of exotic phases at several times nuclear saturation density are outlined.
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