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Constraining the dense matter equation of state with new NICER mass-radius measurements and new chiral effective field theory inputs
Nathan Rutherford, Melissa Mendes, Isak Svensson, Achim Schwenk, Anna L. Watts, Kai Hebeler, Jonas Keller, Chanda Prescod-Weinstein, Devarshi Choudhury, Geert Raaijmakers, Tuomo Salmi, Patrick Timmerman, Serena Vinciguerra, Sebastien Guillot, James M. Lattimer
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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 (χEFT), 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 1.42M_⊙ 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 $\chi$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 1.4M_⊙ (2.0M_⊙) neutron star to be constrained to the 95% credible ranges $12.28^{+0.50}_{-0.76}\,$km ($12.33^{+0.70}_{-1.34}\,$km) for the PP model and $12.01^{+0.56}_{-0.75}\,$km ($11.55^{+0.94}_{-1.09}\,$km) for the CS model. The maximum neutron star mass is predicted to be $2.15^{+0.14}_{-0.16}\,M_⊙and2.08^(+0.28)_(-0.16)M_\odot$ for the PP and CS model, respectively. We explore the sensitivity of our results to different orders and different densities up to which χEFT 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 2M_⊙ and 1.4M_⊙ neutron stars within our EOS inference.
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