2021ApJ...918L..28M

PSR J0740+6620 has a gravitational mass of 2.08 ± 0.07 M_☉, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-ray observations. We find that the equatorial circumferential radius of PSR J0740+6620 is 13.7_–1.5^+2.6 km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030+0451, the masses of two other ∼2 M_☉ pulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation-of-state modeling, and find consistent results at ∼1.5-5 times nuclear saturation density. For a given framework, when all measurements are included, the radius of a 1.4 M_☉ neutron star is known to ±4% (68% credibility) and the radius of a 2.08 M_☉ neutron star is known to ±5%. The full radius range that spans the ±1σ credible intervals of all the radius estimates in the three frameworks is 12.45 ± 0.65 km for a 1.4 M_☉ neutron star and 12.35 ± 0.75 km for a 2.08 M_☉ neutron star.