Large and massive neutron stars: implications for the sound speed in dense QCD

Christian Drischler, Sophia Han, Sanjay Reddy.


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 [Miller et al., Astrophys. J. Lett. 918, L28 (2021); Riley et al., Astrophys. J. Lett. 918, L27 (2021)] suggests that PSR J0740+6620 has a radius in the range of R_{2.0} \approx (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 (\chiEFT) 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, \textbf{min}\{c^2_{s, {\rm max}}\}, increases rapidly with the radius of massive neutron stars. If \chiEFT remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, R_{2.0}\geqslant 13 km requires \textbf{min}\{c^2_{s, {\rm max}}\} \geqslant 0.562 and 0.442 at the 68\% and 95\% credibility level, respectively.