Constraining the dense matter equation of state with new NICER mass-radius measurements and new chiral effective field theory inputs

Abstract

Pulse profile modeling of X-ray data from NICER is now enabling precision inference of neutron star mass and radius. Combined with nuclear physics constraints from chiral effective field theory ($\ddot{I}‡$EFT), and masses and tidal deformabilities inferred from gravitational wave detections of binary neutron star mergers, this has lead to a steady improvement in our understanding of the dense matter equation of state (EOS). Here we consider the impact of several new results: the radius measurement for the 1.42$M_{\odot }$ pulsar PSR J0437$-$4715 presented by Choudhury et al. (2024), updates to the masses and radii of PSR J0740$+$6620 and PSR J0030$+$0451, and new $\ddot{I}‡$EFT results for neutron star matter up to 1.5 times nuclear saturation density. Using two different high-density EOS extensions -- a piecewise-polytropic (PP) model and a model based on the speed of sound in a neutron star (CS) -- we find the radius of a 1.4$M_{\odot }$ (2.0$M_{\odot }$) neutron star to be constrained to the 95% credible ranges $12.28_{-0.76}^{+0.50}$km ($12.33_{-1.34}^{+0.70}$km) for the PP model and $12.01_{-0.75}^{+0.56}$km ($11.55_{-1.09}^{+0.94}$km) for the CS model. The maximum neutron star mass is predicted to be $2.15_{-0.16}^{+0.14}$$M_{\odot }$ and $2.08_{-0.16}^{+0.28}$$M_{\odot }$ for the PP and CS model, respectively. We explore the sensitivity of our results to different orders and different densities up to which $\ddot{I}‡$EFT is used, and show how the astrophysical observations provide constraints for the pressure at intermediate densities. Moreover, we investigate the difference $R_{2.0}-R_{1.4}$ of the radius of 2$M_{\odot }$ and 1.4$M_{\odot }$ neutron stars within our EOS inference.

23 pages, 15 figures; published version