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If we ignore the rotation issue (as Pela pointed out, the correction is <1%), then we can approximate the spacetime curvature inside the neutron star using the interior Schwarzschild metric and use the exterior one for the outside. The $r$ coordinate is not the radial distance as one would expect, but defined in terms of $r=$ constant circles having ...
The inferred radius by a distant observer is given by $$R_{\infty} = \frac{R}{\sqrt{1- R_s/R}},$$ where $R_s = 2GM/c^2$, $M$ is the gravitational mass and $R$ is the radius measured at the surface. The fact that $R_{\infty} > R$ is because an observer can see more than 50 per cent of the neutron star surface. See https://physics.stackexchange.com/...