The evolution of binary neutron star post-merger remnants: a review

TL;DR

This review summarizes observational and theoretical knowledge on binary neutron star post-merger remnants, discussing their evolution, detection prospects, and implications for nuclear physics and astrophysics.

Contribution

It provides a comprehensive synthesis of current understanding and future prospects regarding the evolution and observation of neutron star merger remnants.

Findings

Evidence suggests many mergers form long-lived neutron stars.

Post-merger remnants' evolution impacts gravitational wave detection.

Electromagnetic observations can probe nuclear matter under extreme conditions.

Abstract

Two neutron stars merge somewhere in the Universe approximately every 10 seconds, creating violent explosions observable in gravitational waves and across the electromagnetic spectrum. The transformative coincident gravitational-wave and electromagnetic observations of the binary neutron star merger GW170817 gave invaluable insights into these cataclysmic collisions, probing bulk nuclear matter at supranuclear densities, the jet structure of gamma-ray bursts, the speed of gravity, and the cosmological evolution of the local Universe, among other things. Despite the wealth of information, it is still unclear when the remnant of GW170817 collapsed to form a black hole. Evidence from other short gamma-ray bursts indicates a large fraction of mergers may form long-lived neutron stars. We review what is known observationally and theoretically about binary neutron star post-merger remnants. From a theoretical perspective, we review our understanding of the evolution of short- and long-lived merger remnants, including fluid, magnetic-field, and temperature evolution. These considerations impact prospects of detection of gravitational waves from either short- or long-lived neutron star remnants which potentially allows for new probes into the hot nuclear equation of state in conditions inaccessible in terrestrial experiments. We also review prospects for determining post-merger physics from current and future electromagnetic observations, including kilonovae and late-time x-ray and radio afterglow observations.