The Radius of PSR J0740+6620 from NICER with NICER Background Estimates

TL;DR

This study refines the neutron star radius and mass estimates of PSR J0740+6620 using NICER background data, confirming previous results and revealing a non-antipodal hot spot configuration.

Contribution

It introduces a background-based Bayesian analysis method for neutron star parameter estimation, providing an independent validation of prior joint fits.

Findings

Neutron star radius estimates are consistent with previous results.

Hot spot configuration shows significant deviation from antipodality.

Inferred offset angle exceeds 25 degrees with high probability.

Abstract

We report a revised analysis for the radius, mass, and hot surface regions of the massive millisecond pulsar PSR J0740+6620, studied previously with joint fits to NICER and XMM-Newton data by Riley et al. (2021) and Miller et al. (2021). We perform a similar Bayesian estimation for the pulse-profile model parameters, except that instead of fitting simultaneously the XMM-Newton data, we use the best available NICER background estimates to constrain the number of photons detected from the source. This approach eliminates any potential issues in the cross-calibration between these two instruments, providing thus an independent check of the robustness of the analysis. The obtained neutron star parameter constraints are compatible with the already published results, with a slight dependence on how conservative the imposed background limits are. A tighter lower limit causes the inferred radius to increase, and a tighter upper limit causes it to decrease. We also extend the study of the inferred emission geometry to examine the degree of deviation from antipodality of the hot regions. We show that there is a significant offset to an antipodal spot configuration, mainly due to the non-half-cycle azimuthal separation of the two emitting spots. The offset angle from the antipode is inferred to be above 25 degrees with 84% probability. This seems to exclude a centered-dipolar magnetic field in PSR J0740+6620.