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Neutron star radius measurement from the ultraviolet and soft X-ray thermal emission of PSR J0437-4715

We analyzed the thermal emission from the entire surface of the millisecond pulsar PSR J0437-4715 observed in the ultraviolet and soft X-ray bands. For this, we calculated non-magnetized, partially ionized atmosphere models of hydrogen, helium, and iron compositions and included plasma frequency effects that may affect the emergent spectrum. This is particularly true for the coldest atmospheres composed of iron (up to a few percent changes in the soft X-ray flux). Employing a Markov chain Monte Carlo method, we found that the spectral fits favour a hydrogen atmosphere, disfavour a helium composition and rule out iron atmosphere and blackbody models. By using a Gaussian prior on the dust extinction, based on the latest 3D map of Galactic dust, and accounting for the presence of hot polar caps found in previous work, we found that the hydrogen atmosphere model results in a well-constrained neutron star radius $R_\mathrm{NS} = 13.6^{+0.9}_{-0.8}~\mathrm{km}$ and bulk surface temperature $T^{\infty}_\mathrm{eff}=\left(2.3\pm0.1\right)\times10^5~\mathrm{K}$. This relatively large radius favours a stiff equation of state and disfavours a strange quark composition inside neutron stars.