Constraining and applying a generic high-density equation of state

TL;DR

This paper explores the CSS parametrization of high-density matter equations of state, applying it to the FCM model of quark matter, and uses neutron star observations to constrain the model parameters.

Contribution

It demonstrates how observational data constrains CSS parameters and maps the FCM model onto the CSS framework, revealing limitations on high-density equations of state.

Findings

Observation of 2 solar mass neutron stars constrains CSS parameters.

FCM equation of state can be accurately represented by CSS.

FCM restricts the CSS parameter space to a limited subspace.

Abstract

We discuss the "constant speed of sound" (CSS) parametrization of the equation of state of high-density matter and its application to the field correlator method (FCM) model of quark matter. We show how observational constraints on the maximum mass and typical radius of neutron stars are expressed as constraints on the CSS parameters. We find that the observation of a $2\,M_{\odot}$ star already severely constrains the CSS parameters, and is particularly difficult to accommodate if the squared speed of sound in the high-density phase is assumed to be around $1/3$ or less. We show that the FCM equation of state can be accurately represented by the CSS parametrization, which assumes a sharp transition to a high-density phase with density-independent speed of sound. We display the mapping between the FCM and CSS parameters, and see that FCM only allows equations of state in a restricted subspace of the CSS parameters.