The Aftermath of Neutron Star Mergers

Neutron stars (NS) are the collapsed cores of supermassive giant stars that contain between 10 and 25 solar masses. Aside from black holes, they are the densest objects in the Universe. Their journey from a main sequence star to a collapsed stellar remnant is a fascinating scientific story.

Sometimes, a binary pair of NS will merge, and what happens then is equally as fascinating.

When two NS merge, a remnant is created that either becomes a black hole or a neutron star, with the black hole being the most common result. But the eventual remnant is just part of the story. There’s a lot going on in the extreme environment created by the merger.

NS mergers can almost instantaneously create extremely powerful magnetic fields trillions of times stronger than Earth’s. They can create short gamma-ray bursts (GRBs). They create kilonovae. They create such an extreme environment that the elusive r-process, or rapid neutron capture process, can occur. The r-process is responsible for a large number of the stable element isotopes heavier than iron, including gold, platinum, and other precious metals.

New research in The Astrophysical Journal examines this extreme environment to see how the interacting forces create a remnant. Its title is “Ab-initio General-relativistic Neutrino-radiation Hydrodynamics Simulations of Long-lived Neutron Star Merger Remnants to Neutrino Cooling Timescales.” The authors are David Radice and Sebastiano Bernuzzi, both from Pennsylvania State University.

The authors say that this is the first ab-initio study into NS mergers. Ab-initio means ‘from the beginning’ in Latin. It means that their simulations are based directly on the fundamental laws of nature and don’t include empirical data. These types of simulations require extremely high levels of computing power, but the payoff is in their predictive power. Ab-initio studies can reveal aspects of complex systems that are extremely difficult to study experimentally. General-relativistic means the simulations incorporate Einstein’s theory of general relativity, which is critical for describing the extreme gravity near neutron stars.

“Despite its astrophysical relevance, the evolution of long-lived NS merger remnants past the GW-dominated phase of their evolution is poorly understood,” the authors write.

The researchers simulated the mergers of a pair of neutron stars with 1.35 solar masses each. The initial distance between the two was a mere 50 km (30 mi). The simulations covered the last ~six orbits prior to the merger and extended to more than ~100 ms after the merger.

“The research explored neutron stars’ early evolution, just moments after they were created,” the authors write. “This research is a starting point for identifying the astronomical signals that could help answer questions about neutron stars and black hole formation.”

The first phase of a neutron star merger, after the inspiral, is the gravitational wave (GW) phase. It lasts until about 20 milliseconds after the merger. By releasing GWs, the neutron star releases some of the merger’s energy.

The next phase is the neutrino cooling phase, and it’s the focus of this work. “We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (?20 ms postmerger),” the authors write.

This figure shows the possible stages of a neutron star merger. It doesn’t show the neutrino cooling phase but does show the viscous phase. Viscosity arises in the remnant due to turbulence and plays key roles in mass ejection and the merger’s outcome: usually a black hole but sometimes a stable NS. Image Credit: Radice D et al. 2020.

Neutrinos are elusive particles that are electrically neutral and have very small masses. According to some research, about 400 billion neutrinos pass through every person on Earth each second. Despite their lack of interaction, neutrinos do carry energy away from the merger, and their energy level depends on the process that formed them. Over time, that energy decays.

A neutron star merger usually creates a black hole remnant. But sometimes, it creates another neutron star called an RMNS, or remnant massive neutron star.

“The neutrino luminosities decay more slowly, so 10–20 ms after merger neutrinos, they become the dominant mechanism through which energy is lost by the RMNS,” the authors write.

This figure from the research shows the GW (red) and neutrino (blue) cooling timescales. About 10 ms after the merger, neutrino radiation becomes the dominant mechanism in the evolution of the remnant. Image Credit: Radice et al. 2024.

The simulations show that the RMNS is different than the protoneutron stars created when massive stars collapse.

The merger creates a dense gas of electron antineutrinos in the RMNS’s outer core. This correlates with hot spots on the outer core. The RMNS is also stable against convection despite the surface being hotter than the core. If there were convective instabilities, they could trigger more GW emissions, but according to the authors, the simulations didn’t show that. “We find no evidence for a revival of the GW signal due to convective instabilities,” they write.

Some research shows that merging NSs are the sources of short gamma-ray bursts (SGRBs.) But for that to happen, the magnetic field needs to somehow escape the remnant and form larger magnetic fields. “If RMNSs are a viable central engine for SGRBs, then the field needs somehow to bubble out of the remnant and form large-scale magnetic structures,” the authors write. But the RMNS’s stability seems to rule that out. “However, our simulations indicate that the RMNS is stably stratified, so it remains unclear how the magnetic fields can emerge from it,” the authors explain.

The merger also creates a massive accretion disk in its outer core.

“A massive accretion disk is formed by the ejection of material squeezed out of the collisional interface between the two stars, forming a massive disk in the first ?20 ms after the merger,” the researchers explain. This disk carries a large portion of the merger’s angular momentum. This allows the RMNS to settle into a fairly stable equilibrium within one of several possible stable configuration regions in the disk.

Illustration showing the merger of two neutron stars. Credit: NASA’s Goddard Space Flight Center/CI Lab

Stable neutron stars are far less common outcomes of mergers than black holes. It only occurs if the combined mass is below a maximum stable mass. But some of the details of how this happened have been obscured.

“These findings reveal a central object surrounded by a rapidly rotating ring of hot matter. If these remnants avoid collapse, scientists expect that they release the majority of their internal energy within seconds of when they form,” the authors write.

Estimates show that as few as 10% of neutron star mergers result in RMNSs, so they’re comparatively rare. By exploring the early evolution of RMNSs, this research has established a starting point for identifying the astronomical signals that can tell scientists more about neutron star mergers and how black holes are created from mergers.

By opening a new window into the fractions of a second that follow a merger, the researchers have also shown the forces involved in creating a very rare object: a stable, remnant massive neutron star.

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