A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437$\unicode{x2013}$4715

TL;DR

This paper uses NICER X-ray data and Bayesian inference to accurately determine the mass, radius, and hot spot properties of the nearby millisecond pulsar PSR J0437–4715, providing insights into neutron star structure and magnetic field geometry.

Contribution

It presents the first detailed Bayesian modeling of NICER data for PSR J0437–4715, combining radio timing priors and hot spot geometry to constrain neutron star properties.

Findings

Mass estimated at 1.418 ± 0.037 M☉

Radius estimated at 11.36^{+0.95}_{-0.63} km

Hot regions are non-antipodal, indicating complex magnetic fields

Abstract

We report Bayesian inference of the mass, radius and hot X-ray emitting region properties - using data from the Neutron Star Interior Composition ExploreR (NICER) - for the brightest rotation-powered millisecond X-ray pulsar PSR J0437$\unicode{x2013}$4715. Our modeling is conditional on informative tight priors on mass, distance and binary inclination obtained from radio pulsar timing using the Parkes Pulsar Timing Array (PPTA) (Reardon et al. 2024), and we use NICER background models to constrain the non-source background, cross-checking with data from XMM-Newton. We assume two distinct hot emitting regions, and various parameterized hot region geometries that are defined in terms of overlapping circles; while simplified, these capture many of the possibilities suggested by detailed modeling of return current heating. For the preferred model identified by our analysis we infer a mass of $M = 1.418 \pm 0.037$ M$_\odot$ (largely informed by the PPTA mass prior) and an equatorial radius of $R = 11.36^{+0.95}_{-0.63}$ km, each reported as the posterior credible interval bounded by the 16% and 84% quantiles. This radius favors softer dense matter equations of state and is highly consistent with constraints derived from gravitational wave measurements of neutron star binary mergers. The hot regions are inferred to be non-antipodal, and hence inconsistent with a pure centered dipole magnetic field.