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Simulations Describe the Aftermath of Neutron Star Collisions

A study led by Pennsylvania State University physicists, published in the journal Physical Reviews Letters, provides new simulations showing that neutrinos released during these catastrophic events are momentarily out of thermodynamic equilibrium with the cold cores of the merging stars.

Volume rendering of density in a simulation of a binary neutron star merger. New research shows that neutrinos created in the hot interface between the merging stars can be briefly trapped and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. Image Credit: David Radice, Penn State

When stars collapse, they leave behind neutron stars, which are extremely dense yet small and cold. If two stars collapse close together, the remaining binary neutron stars spiral in and eventually collide, and the interface where the two stars begin to merge becomes extremely hot

New simulations of these events show that hot neutrinos — tiny, essentially massless particles that rarely interact with other matter — produced during the collision can be temporarily trapped at these interfaces and remain out of equilibrium with the merging stars’ cold cores for 2 to 3 milliseconds.

During this period, the simulations reveal that neutrinos can weakly interact with star matter, helping to bring the particles back to equilibrium and providing new insights into the physics of these intense processes.

For the first time in 2017, we observed here on Earth signals of various kinds, including gravitational waves, from a binary neutron star merger. This led to a huge surge of interest in binary neutron star astrophysics. There is no way to reproduce these events in a lab to study them experimentally, so the best window we have into understanding what happens during a binary neutron star merger is through simulations based on math that arises from Einstein’s theory of general relativity.

Pedro Luis Espino, Study Lead and Postdoctoral Researcher, Pennsylvania State University

Neutron stars are named because they are assumed to be almost entirely made up of neutrons, which, combined with positively charged protons and negatively charged electrons, make up atoms. Their enormous density—only black holes are smaller and denser—is believed to compress protons and electrons, combining them into neutrons.

A typical neutron star is merely tens of kilometers across but has around one-and-a-half times the mass of our Sun, which is about 1.4 million kilometers across. A teaspoon of neutron star material could weigh tens or hundreds of millions of tons, just like a mountain.

Neutron stars before the merger are effectively cold, while they may be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system. As they collide, they can become really hot, the interface of the colliding stars can be heated up to temperatures in the trillions of degrees Kelvin. However, they are so dense that photons cannot escape to dissipate the heat; instead, we think they cool down by emitting neutrinos.

David Radice, Study Lead and Assistant Professor, Astronomy and Astrophysics, Eberly College of Science, Pennsylvania State University

According to the researchers, neutrinos are produced via collisions between neutrons in stars, which are blown apart into protons, electrons, and neutrinos. What occurs in the immediate aftermath of a collision has long been a mystery in astrophysics.

To address this issue, the researchers developed simulations that required vast amounts of computing power to imitate the merging of binary neutron stars and their associated physics.

The models demonstrated for the first time that the heat and density of the merger could capture neutrinos, although temporarily. The hot neutrinos are out of balance with the stars' still cool cores, allowing them to interact with the matter of the stars.

Radice added, “These extreme events stretch the bounds of our understanding of physics and studying them allows us to learn new things. The period where the merging stars are out of equilibrium is only 2 to 3 milliseconds, but like temperature, time is relative here, the orbital period of the two stars before the merge can be as little as 1 millisecond. This brief out-of-equilibrium phase is when the most interesting physics occurs, once the system returns to equilibrium, the physics is better understood.

The researchers highlighted that the specific physical interactions that occur during the merger might influence the types of signals that can be detected on Earth due to binary star mergers.

Espino further stated, “How the neutrinos interact with the matter of the stars and eventually are emitted can impact the oscillations of the merged remnants of the two stars, which in turn can impact what the electromagnetic and gravitation wave signals of the merger look like when they reach us here on Earth.

Next-generation gravitation-wave detectors could be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role allowing us to get insight into these extreme events while informing future experiments and observations in a kind of feedback loop,” Espino added.

The study team comprises Espino and Radice, as well as postdoctoral scholars Peter Hammond and Rossella Gamba from Penn State, Sebastiano Bernuzzi, Francesco Zappa, and Luís Felipe Longo Micchi from Friedrich-Schiller-Universität Jena in Germany, and Albino Perego from Università di Trento in Italy.

This research was funded by the US National Science Foundation, US Department of Energy, Office of Science, Division of Nuclear Physics, the Deutsche Forschungsgemeinschaft, and the European Union's Horizon 2020 and Europe Horizon initiatives.

Simulations were run on the Bridges2, Expanse, Frontera, and Perlmutter supercomputers. The study used the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the United States Department of Energy’s Office of Science.

The authors acknowledged the Gauss Centre for Supercomputing e.V. for funding this study by giving computing time on the GCS Supercomputer SuperMUC-NG at the Leibniz Supercomputing Centre.

Journal Reference:

Espino, P. L., et al. (2024) Neutrino Trapping and Out-of-Equilibrium Effects in Binary Neutron-Star Merger Remnants. Physical Reviews Letters. doi:10.1103/PhysRevLett.132.211001

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