Astrophysical constraints on compact objects in 4D Einstein-Gauss-Bonnet gravity

TL;DR

This paper explores how a modified gravity theory affects the properties of compact objects like neutron stars and black holes, revealing potential observational signatures and constraints on deviations from general relativity.

Contribution

It provides the first detailed analysis of neutron stars and black holes in 4D Einstein-Gauss-Bonnet gravity, showing how deviations from GR influence their properties and observational compatibility.

Findings

Neutron stars can match black hole limits, closing the mass gap.

The secondary in GW190814 could be a neutron star under this theory.

A universal endpoint exists for neutron star sequences, independent of the equation of state.

Abstract

We study the properties of compact objects in a particular 4D Horndeski theory originating from higher dimensional Einstein-Gauss-Bonnet gravity. Remarkably, an exact vacuum solution is known. This compact object differs from general relativity mostly in the strong field regime. We discuss some properties of black holes in this framework and investigate in detail the properties of neutron stars, both static and in slow rotation. We find that for relatively modest deviations from general relativity, the secondary object in GW190814 is compatible with being a slowly-rotating neutron star, without resorting to very stiff or exotic equations of state. Remarkably, the equilibrium sequence of neutron stars matches asymptotically to the black hole limit, completetly closing the mass gap between neutron stars and black holes of same radius, although the stability of equilibrium solutions has yet to be determined. As a consequence, there exists a universal endpoint for the neutron star sequence, independent of the equation of state. In light of our results and of current observational constraints, we discuss specific constraints on the coupling constant that parametrizes deviations from general relativity in this theory.