A NICER view of PSR J0030+0451: Implications for the dense matter equation of state

TL;DR

This study uses NICER X-ray data and Bayesian methods to infer the dense matter equation of state from the pulsar PSR J0030+0451's mass and radius, integrating nuclear physics and astrophysical constraints.

Contribution

It introduces a Bayesian framework combining NICER data with nuclear physics models to constrain the dense matter EOS using pulsar observations.

Findings

NICER data is consistent with high-density EOS models.

Current uncertainties limit the information gain from PSR J0030+0451.

EOS constraints are improved by combining astrophysical and nuclear physics data.

Abstract

Both the mass and radius of the millisecond pulsar PSR J0030+0451 have been inferred via pulse-profile modeling of X-ray data obtained by NASA's NICER mission. In this Letter we study the implications of the mass-radius inference reported for this source by Riley et al. (2019) for the dense matter equation of state (EOS), in the context of prior information from nuclear physics at low densities. Using a Bayesian framework we infer central densities and EOS properties for two choices of high-density extensions: a piecewise-polytropic model and a model based on assumptions of the speed of sound in dense matter. Around nuclear saturation density these extensions are matched to an EOS uncertainty band obtained from calculations based on chiral effective field theory interactions, which provide a realistic description of atomic nuclei as well as empirical nuclear matter properties within uncertainties. We further constrain EOS expectations with input from the current highest measured pulsar mass; together, these constraints offer a narrow Bayesian prior informed by theory as well as laboratory and astrophysical measurements. The NICER mass-radius likelihood function derived by Riley et al. (2019) using pulse-profile modeling is consistent with the highest-density region of this prior. The present relatively large uncertainties on mass and radius for PSR J0030+0451 offer, however, only a weak posterior information gain over the prior. We explore the sensitivity to the inferred geometry of the heated regions that give rise to the pulsed emission, and find a small increase in posterior gain for an alternative (but less preferred) model. Lastly, we investigate the hypothetical scenario of increasing the NICER exposure time for PSR J0030+0451.