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

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 gravitationalwave 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.

Code Comparison for Fast Flavor Instability Simulation

“…Neutrinos energize the explosions of massive stars and drive outflows from neutron star mergers and protoneutron stars. In both systems, interactions between neutrinos and outflows determine the elements that form and enrich the universe [1][2][3][4]. Furthermore, electron flavor neutrinos and antineutrinos interact more strongly with matter than other flavors due to the large masses of muons and taons, and it is effectively only this flavor of neutrino that is able to convert neutrons to protons and vice versa.…”

Frequency deviations in universal relations of isolated neutron stars and postmerger remnants

“…At a frequency f peak in the range of a few kHz, the dominant fluid oscillation of the merger remnant is an efficient emitter of GWs, see e.g. [2][3][4][5][6][7][8][9][10][11][12] as well as the reviews [13][14][15][16][17][18][19] and references therein. Hence, it is an important target of current GW searches.…”

Models of binary neutron star remnants with tabulated equations of state

“…However, the loss of angular momentum, due to GW emission, as well as dissipative effects (e.g. shear viscosity, magnetic breaking and effective viscosity due to the development of the magneto-rotational instability, see Shibata & Hotokezaka (2019); Ciolfi (2020); Sarin & Lasky (2020); Ruiz et al (2021) for recent reviews and also Radice 2020) will ultimately lead to a delayed collapse to a black hole. A remnant with mass 𝑀 max,rot < 𝑀 < 𝑀 max , where 𝑀 max , which is the maximum mass of a nonrotating star, will be long-lived, spinning down on the timescale of electromagnetic emission, before reaching the axisymmetric instability limit.…”