Extreme Matter meets Extreme Gravity: Ultra-heavy neutron stars with crossovers and first-order phase transitions

TL;DR

This paper explores how non-smooth features in the speed of sound within neutron star matter, caused by phase transitions or crossovers, can lead to the existence of ultra-heavy neutron stars and affect various astrophysical observables.

Contribution

It demonstrates that non-smooth structures in the speed of sound can produce ultra-heavy neutron stars consistent with current observations and analyzes their impact on observable properties.

Findings

Ultra-heavy neutron stars are possible with non-smooth speed of sound features.

Non-smooth structures influence stellar radii, tidal deformability, and other observables.

Results align with recent gravitational wave and X-ray data.

Abstract

The speed of sound of the matter within neutron stars may contain non-smooth structure related to first-order phase transitions or or crossovers. Here we investigate what are the observable consequences of structure, such as bumps, spikes, step functions, plateaus, and kinks. One of the main consequences is the possibility of ultra-heavy neutron stars, i.e.~stars with masses significantly heavier than two solar masses. These stars pass all observational and theoretical constraints, including those imposed by recent LIGO/Virgo gravitational-wave observations and NICER X-ray observations. We thoroughly investigate other consequences of this structure in the speed of sound to develop an understanding of how non-smooth features affect astrophysical observables, such as stellar radii, tidal deformability, moment of inertia, and Love number. Our results have important implications to future gravitational wave and X-ray observations of neutron stars and their impact in nuclear astrophysics.