Evidence for a hard equation of state in the cores of neutron stars

TL;DR

This paper provides evidence that the core of neutron stars has a stiff, or hard, equation of state, based on modeling and observational data, indicating high adiabatic indices and densities.

Contribution

It introduces a parametrization of the neutron star core equation of state and constrains the adiabatic index and density using observational data.

Findings

Adiabatic index Gamma > 2.2 in neutron star cores

Central density between 10^15 and 10^16 g/cm^3

Neutrino flux suggests even higher Gamma around 3.5

Abstract

The equation of state for matter with energy density above 2 x10^14 g/cm^3 is parametrized by P = kN^Gamma, where N is the number density, Gamma is the adiabatic index, and k a constant. Using this scheme to generate thousands of models, together with data on neutron star masses, it is found, for a core region with a constant adiabatic index, that the central density must satisfy 10^15 gm/cm^3 < rho_c < 10^16 gm/cm^3, with Gamma > 2.2. Further preliminary results indicate, based on the observed neutrino flux from supernova 1987a, that this number must be considerably higher, on the order of 3.5. These results provide evidence for a hard equation of state in the cores of neutron stars.