The Radius of PSR J0740+6620 from NICER and XMM-Newton Data

PSR J0740$+$6620 has a gravitational mass of $2.08\pm 0.07~M_\odot$, which is the highest reliably determined any neutron star. As result, measurement its radius will provide unique insight into properties star core matter at high densities. Here we report based on fits rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) observations. We find that equatorial circumferential $13.7^{+2.6}_{-1.5}$ km (68%). apply our measurement, combined with previous NICER J0030$+$0451, masses two other $\sim 2~M_\odot$ pulsars, tidal deformability constraints from wave events, three different frameworks for equation state modeling, consistent results 1.5-3$ times nuclear saturation density. For given framework, when all measurements are included $1.4~M_\odot$ known $\pm 4$% (68% credibility) $2.08~M_\odot$ 5$%. The full range spans 1\sigma$ credible intervals estimates in $12.45\pm 0.65$ $12.35\pm 0.75$