Astrophysical constraints on the cold equation of state of the strongly interacting matter

TL;DR

This paper uses astrophysical observations, including neutron star measurements and gravitational wave data, to constrain the equation of state of cold, dense, strongly interacting matter.

Contribution

It combines multiple observational constraints with perturbative QCD calculations to refine the EOS parameters for dense matter.

Findings

Neutron star observations significantly restrict the EOS of dense matter.

Massive neutron stars and tidal deformability constraints are most restrictive.

NICER measurements have large uncertainties but still contribute to EOS constraints.

Abstract

At present, the only experimental access to the properties of cold, dense strongly interacting matter is provided by astrophysical observations. Neutron stars are the only known systems in the Universe that reach densities several times higher than normal nuclear density at nearly zero temperature, making them unique laboratories for studying dense matter. Since most neutron star observables are sensitive to the equation of state (EOS), observational data place stringent constraints on the EOS of strongly interacting matter. In this work, we investigate constraints arising from perturbative QCD calculations at asymptotically high densities ($\rho \approx 40 \rho_0$), the mass of the heaviest observed neutron star (a black widow pulsar), NICER mass-radius measurements, and the tidal deformability inferred from the binary neutron star merger GW170817. We parametrize the EOS and allow its parameters to vary freely, using observational data to constrain the admissible parameter space. We find that neutron star observations significantly restrict the EOS of dense strongly interacting matter. While NICER has already provided measurements for five pulsars, the associated uncertainties remain relatively large. Within our modeling framework, we find that the existence of very massive neutron stars and constraints on the tidal deformability provide the most restrictive constraints on the EOS.