Gravitational-Wave and X-ray Probes of the Neutron Star Equation of State

TL;DR

This paper reviews how gravitational wave and X-ray observations are advancing our understanding of neutron star interiors and the exotic matter they may contain, bridging astrophysics and nuclear physics.

Contribution

It synthesizes current observational insights into neutron star physics and discusses future prospects for constraining the dense matter equation of state.

Findings

Gravitational wave data constrains neutron star radii and masses.

X-ray observations provide insights into neutron star surface and interior composition.

Future observations will further elucidate the presence of exotic matter in neutron star cores.

Abstract

Neutron stars are a remarkable marriage of Einstein's theory of general relativity with nuclear physics. Their interiors harbor extreme matter that cannot be probed in the laboratory. At such high densities and pressures, their cores may consist predominantly of exotic matter such as free quarks or hyperons. Gravitational wave observations from the Laser Interferometer Gravitational-wave Observatory (LIGO) and from other interferometers, and X-ray observations from the Neutron Star Interior Composition Explorer (NICER), are beginning to pierce through the veil. These observations provide information about neutron star cores, and therefore, about the physics that makes such objects possible. In this review, we discuss what we have learned about the physics of neutron stars from gravitational wave and X-ray observations. We focus on what has been observed with certainty and what should be observable in the near future, with an eye out for the physics that these new observations will teach us.