A NICER View of PSR J1231$-$1411: A Complex Case

TL;DR

This study uses NICER and XMM-Newton data to perform Bayesian inference on the neutron star PSR J1231$-$1411, estimating its mass and radius while exploring emission geometry and prior influences.

Contribution

It applies a Bayesian modeling approach to a new pulsar, PSR J1231$-$1411, with detailed analysis of priors and emission geometry, expanding neutron star EOS constraints.

Findings

Estimated radius: 12.6 ± 0.3 km with constrained prior.

Estimated mass: 1.04_{-0.03}^{+0.05} M_sun with constrained prior.

Identified complex hot spot geometry with noncircular regions.

Abstract

Recent constraints on neutron star mass and radius have advanced our understanding of the equation of state (EOS) of cold dense matter. Some of them have been obtained by modeling the pulses of three millisecond X-ray pulsars observed by the Neutron Star Interior Composition Explorer (NICER). Here, we present a Bayesian parameter inference for a fourth pulsar, PSR J1231$-$1411, using the same technique with NICER and XMM-Newton data. When applying a broad mass-inclination prior from radio timing measurements and the emission region geometry model that can best explain the data, we find likely converged results only when using a limited radius prior. If limiting the radius to be consistent with the previous observational constraints and EOS analyses, we infer the radius to be $12.6 \pm 0.3$ km and the mass to be $1.04_{-0.03}^{+0.05}$ $M_\odot$, each reported as the posterior credible interval bounded by the $16\,\%$ and $84\,\%$ quantiles. If using an uninformative prior but limited between $10$ and $14$ km, we find otherwise similar results, but $R_{\mathrm{eq}} = 13.5_{-0.5}^{+0.3}$ km for the radius. In both cases, we find a nonantipodal hot region geometry where one emitting spot is at the equator or slightly above, surrounded by a large colder region, and where a noncircular hot region lies close to southern rotational pole. If using a wider radius prior, we only find solutions that fit the data significantly worse. We discuss the challenges in finding the better fitting solutions, possibly related to the weak interpulse feature in the pulse profile.