Implications of NICER for neutron star matter: the QHC21 equation of state

Abstract

The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation in neutron star radii from 1.4M to 2.1M , suggest that the neutron star equation of state remains relatively stiff up to baryon densities n ∼ 2-4 times nuclear saturation density, n 0 -the region where we expect hadronic matter to be undergoing transformation into quark matter. To delineate the physics from the nuclear to the quark matter regimes we use the quark-hadron-crossover (QHC) template to construct an updated equation of state, QHC21. We include nuclear matter results primarily based on chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces, thus covering the range of uncertainties in the nuclear equation of state. To allow for a possible early transition to quark degrees of freedom we begin the crossover regime from nucleons to quarks at 1.5n 0 . The resulting equations of state are stiffer than our earlier QHC19 at 2n 0 , predicting larger radii in substantial agreement with the NICER data, with accompanying peaks in sound velocity at 2-4n 0 . We discuss possible microscopic mechanisms underlying stiffening of the equation of state.

The quarkyonic matter state of neutron stars

“…We are glad to see that the pressure is consistent with the experimental constraints very well [15], neither too soft nor too stiff. But the square of sound velocity gradually approaches the free quark limit 1/3 from below without developing any peak structure, in contrast to other studies [11,23] involving both nucleons and quarks (see the review Ref. [24]).…”

A Stable Sexaquark: Overview and Discovery Strategies

“…A potential astrophysical challenge to the existence of a stable sexaquark is the observation of neutron stars with masses above 2 M . Hyperons, which we know exist, present a similar problem for the existence of massive neutron stars, a problem resolved by quark deconfinement [67,68] at densities below the emergence of a hyperon-dominated phase. Reference [69] takes an empirical approach, allowing for both a deconfined phase and a density dependence of the hadron masses as in [70] and demanding consistency with both the GW170817 constraints on tidal deformability [71] simultaneously with the mass-radius relation of pulsars including the highly constraining NICER results on PSR J0740+6620 [72][73][74].…”

Universal Relations for the Increase in the Mass and Radius of a Rotating Neutron Star

“…We included the following hadronic EOSs: the APR EOS (Akmal et al 1998) includes three-nucleon interactions and special relativistic corrections; the two BBB EOS (Baldo et al 1997) as hadronic examples that have maximum masses smaller than 2 M e ; an example of an EOS that includes hyperons (Table 5.8 of Glendenning 1997), with a maximum mass below 1.6 M e ; the ultra stiff EOS L (Pandharipande & Smith 1975) with a pion condensate. We also include three quark/hadron hybrid EOSs (Alford et al 2005;Kojo et al 2021) and a bare quark star (Alcock et al 1986).…”