Dense baryonic matter: Constraints from recent neutron star observations

Abstract

Updated constraints from neutron star masses and radii impose stronger restrictions on the equation of state for baryonic matter at high densities and low temperatures. The existence of two-solar-mass neutron stars rules out many soft equations of state with prominent "exotic" compositions. The present work reviews the conditions required for the pressure as a function of baryon density in order to satisfy these constraints. Several scenarios for sufficiently stiff equations of state are evaluated. The common starting point is a realistic description of both nuclear and neutron matter based on a chiral effective field theory approach to the nuclear many-body problem. Possible forms of hybrid matter featuring a quark core in the center of the star are discussed using a threeflavor Polyakov-Nambu-Jona-Lasinio (PNJL) model. It is found that a conventional equation of state based on nuclear chiral dynamics meets the astrophysical constraints. Hybrid matter generally turns out to be too soft unless additional strongly repulsive correlations, e.g. through vector current interactions between quarks, are introduced. The extent to which strangeness can accumulate in the equation of state is also discussed.

Functional renormalization group studies of nuclear and neutron matter

“…18 displays a band of calculated FRG-ChNM neutron matter equations of state covering a range of symmetry energies, 29 MeV ≤ E sym ≤ 33 MeV, corresponding to an interval 0.9 fm 2 ≤ G τ ≤ 1.2 fm 2 of short-range isovector-vector couplings. Also shown for comparison are the APR equation of state based on phenomenological potentials [113], results from chiral effective field theory [116], and different QMC computations using phenomenological [114] and chiral potentials [102]. The equations of state obtained in the FRG-extended ChNM model agree quite well with all these results up to densities as high as n = 3 n 0 .…”

“…Note that in contrast to the mean-field approximation which systematically deviates at higher densities, the FRG treatment of fluctua- . Also shown for reference are predictions from ChEFT (full line, [116]), QMC based on realistic potentials (dashed line [114]), QMC based on chiral potentials (dotted line [102]), and the Akmal-Pandharipande-Ravenhall EoS (dashed-dotted line [113]). …”

“…[116,124]), nor do we include other exotic types of matter such as kaon condensates. Hyperons are also not included as they would generally soften the equation of state unless strong additional repulsion is introduced for compensation [125,126].…”

“…Even mass at n ⇠ 5 n 0 is reduced to less than half value, chiral symmetry is still realized in the [46]. Solid curve: ChEFT result [116]. Constraints on radii, "Tr2011" from [131], "HLPS2013" from [118,111], and from two-solar-mass neutron stars [119,120] are shown for guidance.…”

“…The resulting pressure versus energy density curve, P (ε), falls well within the acceptable EoS band constrained by neutron star data as discussed in Refs. [115,116,117,118]. Here we briefly examine how this figures in the mass-radius plot for neutron stars, especially in view of the existence of two heavy n-stars with accurately determined masses: J1614-2230 with M = (1.97 ± 0.04) M [119], and J0348+0432 with M = (2.01 ± 0.04) M [120].…”