Large and massive neutron stars: Implications for the sound speed within QCD of dense matter

The NASA telescope NICER has recently measured x-ray emissions from the heaviest of the precisely known two-solar mass neutron stars, PSR J0740 + 6620. Analysis of the data [Astrophys. J. Lett. 918, L28 (2021), 10.3847/2041-8213/ac089b, Astrophys. J. Lett. 918, L27 (2021), 10.3847/2041-8213/ac0a81] suggests that PSR J0740 + 6620 has a radius in the range of R_2.0≈(11.4 -16.1 ) km at the 68 % credibility level. In this article, we study the implications of this analysis for the sound speed in the high-density inner cores by using recent chiral effective field theory (χ EFT ) calculations of the equation of state at next-to-next-to-next-to-leading order to describe outer regions of the star at modest density. We find that the lower bound on the maximum speed of sound in the inner core, min{c_s^{,max 2}} , increases rapidly with the radius of massive neutron stars. If χ EFT remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, R_2.0⩾13 km requires min{c_s^{,max 2}}⩾0.562 and 0.442 at the 68% and 95% credibility level, respectively.