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

TL;DR

This paper measures the radius of the high-mass neutron star PSR J0740+6620 using X-ray data from NICER and XMM-Newton, providing key insights into neutron star matter at high densities.

Contribution

It presents the first radius measurement of PSR J0740+6620 using combined NICER and XMM-Newton data, and integrates this with other observations to constrain neutron star equations of state.

Findings

Radius of PSR J0740+6620 is approximately 13.7 km with uncertainties.

Combined measurements constrain neutron star radii to within a few kilometers.

Results are consistent across multiple theoretical frameworks for dense matter.

Abstract

PSR J0740$+$6620 has a gravitational mass of $2.08\pm 0.07~M_\odot$, 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 $13.7^{+2.6}_{-1.5}$ km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030$+$0451, the masses of two other $\sim 2~M_\odot$ 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 $\sim 1.5-3$ times nuclear saturation density. For a given framework, when all measurements are included the radius of a $1.4~M_\odot$ neutron star is known to $\pm 4$% (68% credibility) and the radius of a $2.08~M_\odot$ neutron star is known to $\pm 5$%. The full radius range that spans the $\pm 1\sigma$ credible intervals of all the radius estimates in the three frameworks is $12.45\pm 0.65$ km for a $1.4~M_\odot$ neutron star and $12.35\pm 0.75$ km for a $2.08~M_\odot$ neutron star.