Neutron stars (NS) are the collapsed cores of supermassive giant stars containing 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 NS pair merges, and what happens then is equally fascinating.
When two neutron stars merge, they create a remnant that becomes either a black hole or a neutron star, with the black hole being the most common result. But the ultimate remnant is only part of the story. A lot happens in the extreme environment created by the merger.
NS mergers can almost instantly produce extremely strong magnetic fields, trillions of times stronger than Earth's. They can produce short gamma-ray bursts (GRBs). They produce kilonovae. They create an environment so extreme that the elusive r-process, or fast neutron capture process, can take place. The r-process is responsible for a large number of stable isotopes of elements heavier than iron, including gold, platinum, and other precious metals.
New research in the Astrophysical Journal examines this extreme environment to find out how the interacting forces create a remnant. The paper is titled “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 of Pennsylvania State University.
The authors say this is the first ab initio study of neutron star mergers. Ab initio means “from the beginning” in Latin. This means that their simulations are based directly on the fundamental laws of nature and do not include empirical data. These types of simulations require extremely high computational power, but the reward is their predictive power. Ab initio studies can reveal aspects of complex systems that are extremely difficult to study experimentally. General relativistic means that the simulations incorporate Einstein's general theory of relativity, which is crucial for describing the extreme gravity near neutron stars.
“Despite their astrophysical relevance, the evolution of the long-lived remnants of neutron fusions after the global radiation-dominated phase of their evolution is poorly understood,” the authors write.
The researchers simulated the merger of two neutron stars, each with 1.35 solar masses. The initial distance between the two was only 50 km. The simulations covered the last ~6 orbits before the merger and extended to over ~100 ms after the merger.
“The research examined the early evolution of neutron stars, just moments after their formation,” the authors write. “This research is a starting point for identifying the astronomical signals that could help answer questions about neutron stars and the formation of black holes.”
The first phase of a neutron star merger after the orbit is the gravitational wave (GW) phase. It lasts until about 20 milliseconds after the merger. By releasing GWs, the neutron star releases part of the energy of the merger.
The next phase is the neutrino cooling phase, and it is the focus of this work. “We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational wave-dominated phase (?20 ms after the merger),” the authors write.
This image shows the possible stages of a neutron star merger. It does not show the neutrino cooling phase, but the viscous phase. Viscosity is created in the remnant by turbulence and plays a key role in the mass ejection and the result of the merger: 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 little mass. According to some studies, about 400 billion neutrinos pass through every person on Earth every second. Despite their lack of interaction, neutrinos carry energy from the merger, and their energy level depends on the process that created them. Over time, this energy decreases.
When two neutron stars merge, the result is usually a black hole remnant. Sometimes, however, another neutron star is formed, a so-called RMNS (Remnant Massive Neutron Star).
“The neutrino luminosity decays more slowly, becoming the dominant mechanism by which the RMNS loses energy 10–20 ms after neutrino fusion,” the authors write.
This image from the research shows the timescales of GW (red) and neutrino (blue) cooling. Around 10 ms after the merger, neutrino radiation becomes the dominant mechanism in the remnant's evolution. Image credit: Radice et al. 2024.
The simulations show that the RMNS is different from the proton neutron stars that form when massive stars collapse.
The merger creates a dense gas of electron antineutrinos in the outer core of the RMNS. This correlates with hotspots in the outer core. The RMNS is also stable against convection, although the surface is hotter than the core. If there were convective instabilities, these could trigger more GW emissions, but according to the authors, the simulations did not show this. “We find no evidence of a revival of the GW signal due to convective instabilities,” they write.
Some studies show that merging neutron stars are the source of short gamma-ray bursts (SGRBs). However, for this to happen, the magnetic field must somehow escape from the remnant and form larger magnetic fields. “If RMNSs are a viable central driver for SGRBs, then the field must somehow bubble out of the remnant and form large-scale magnetic structures,” the authors write. But the stability of the RMNS seems to rule this out. “However, our simulations show that the RMNS is stably layered, so it remains unclear how the magnetic fields can emerge from it,” the authors explain.
The merger also creates a huge accretion disk in its outer core.
“A massive accretion disk is formed by the ejection of material from the collision surface between the two stars. In the first 20 ms after the merger, a massive disk forms,” the researchers explain. This disk carries a large part of the angular momentum of the merger. This allows the RMNS to settle into a fairly stable equilibrium in one of several possible stable configuration regions in the disk.
Image of the merger of two neutron stars. Image credit: NASA Goddard Space Flight Center/CI Lab
Stable neutron stars are far rarer than black holes in mergers. They only occur when the total mass is below a maximum stable mass. However, some details about this are unclear.
“These results show a central object surrounded by a rapidly rotating ring of hot matter. If these remnants do not collapse, scientists expect them to release most of their internal energy within seconds of their formation,” the authors write.
It is estimated that only 10% of neutron star mergers result in RMNSs, making them relatively rare. By studying the early evolution of RMNSs, this research has provided a starting point for identifying the astronomical signals that can tell scientists more about neutron star mergers and the formation of black holes from mergers.
By opening a new window into the fractions of a second that follow a merger, the researchers also revealed the forces involved in the formation of a very rare object: a stable, remnant massive neutron star.
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