Equation of state constraints from neutron stars

Recent measurements of thermal radiation from neutron stars have suggested a rather broad range of radiation radii (R∞=R/√{1-2GM/Rc ². Sources in M13 and Omega Cen imply R _∞ ∼ 12-14 km, but X7 in 47 Tuc implies R _∞∼16-20 km and RX J1856-3754 R _∞>17 km. If these measurements are all correct, only a limited selection of EOS's could be consistent with them, but a broad range of neutron star masses (up to 2 M_⊙) would also be necessary. The surviving equations of state are incompatible with significant softening above nuclear saturation densities, such as would occur with Boson condensates, a low-density quark-hadron transition, or hyperons. Other potential constraints, such as from QPO's, radio pulsar mass and moment of inertia measurements, and neutron star cooling, are compared.