Large and massive neutron stars: Implications for the sound speed within QCD of dense matter
C. Drischler, Sophia Han, Sanjay Reddy
Abstract
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), Astrophys. J. Lett. 918, L27 (2021)] suggests that PSR J0740 $+$ 6620 has a radius in the range of ${R}_{2.0}\ensuremath{\approx}(11.4--16.1)\phantom{\rule{4pt}{0ex}}\text{km}$ 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 ($\ensuremath{\chi}\mathrm{EFT}$) 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, $min{{c}_{s,\mathrm{max}}^{2}}$, increases rapidly with the radius of massive neutron stars. If $\ensuremath{\chi}\mathrm{EFT}$ remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, ${R}_{2.0}\ensuremath{\geqslant}13$ km requires $min{{c}_{s,\mathrm{max}}^{2}}\ensuremath{\geqslant}0.562$ and 0.442 at the 68% and 95% credibility level, respectively.