During a binary neutron star collision, hot neutrinos can be briefly trapped at the interface and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. This interaction helps steer the particles toward equilibrium and offers new insight into the physics of such mergers. Credit: SciTechDaily.com
New simulations show that neutrinos were created during these cataclysms neutron star the collisions are briefly out of thermodynamic equilibrium with the cool cores of the merging stars.
Recent simulations by Penn State physicists have shown that during binary neutron star mergers, hot neutrinos can be briefly captured and left out of equilibrium, providing new understanding of these cosmic events. This research highlights the role of simulations in studying phenomena that cannot be replicated experimentally.
What happens when neutron stars collide?
When stars collapse, they often leave behind incredibly dense but relatively small and cool remnants called neutron stars. If two stars collapse in close proximity, the remaining binary neutron stars spiral closer and eventually collide, heating the collision point to extreme temperatures.
New simulations of these events show that hot neutrinos—tiny, essentially massless particles that rarely interact with other matter—that are produced during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cool cores of the merging stars for 2 up to 20 minutes. 3 milliseconds. During this time, simulations show that neutrinos can interact weakly with the stars’ mass, helping to drive the particles back to equilibrium – and providing new insight into the physics of these powerful events.
Pioneering simulations of neutron star mergers
A paper describing the simulations by a research team led by Penn State physicists was recently published in the journal Physical review letters.
“For the first time in 2017, we observed signals of various kinds here on Earth, including gravitational wavesfrom binary neutron star mergers,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and University of California, Berkeley, who led the research. “This has led to a huge increase in interest in the astrophysics of binary neutron stars.” There is no way to reproduce these events in the laboratory so that we can study them experimentally, so the best window we have into understanding what happens during binary neutron star mergers is through simulations based on the mathematics that comes from Einstein’s general theory relativity.
Volume density rendering in a simulation of a binary neutron star merger. New research shows that neutrinos produced at the hot interface between merging stars can be briefly captured and remain out of equilibrium with the cool cores of the merging stars for 2 to 3 milliseconds. Credit: David Radice, Penn State
Neutron star composition and collision dynamics
Neutron stars get their name because they are thought to consist almost entirely of neutrons, uncharged particles that, along with positively charged protons and negatively charged electrons, make up atoms. Their incredible density – only black holes are smaller and denser – is thought to squeeze protons and electrons together, fusing them into neutrons. A typical neutron star is only tens of kilometers across, but has about one and a half times the mass of our Sun, which is about 1.4 million kilometers across. A teaspoon of neutron star material can weigh as much as a mountain, tens or hundreds of millions of tons.
“Pre-merger neutron stars are effectively cold, although they can be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” said David Radice, assistant professor of physics and astronomy and astrophysics. at Penn State’s Eberly College of Science and leader of the research team. “As they collide, they can get really hot, the interface of colliding stars can heat 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 by emitting neutrinos.”
Insights from the behavior of neutrinos in stellar mergers
According to the researchers, neutrinos are created during collisions when neutrons in stars collide and are blasted into protons, electrons and neutrinos. What then happens in those first moments after the collision has been an open question in astrophysics.
To try to answer this question, the research team created computationally intensive simulations that model binary neutron star mergers and all the physics involved. The simulations showed for the first time that even neutrinos can be trapped by the heat and density of the merger, even if briefly. Hot neutrinos are out of equilibrium with the still cool stellar cores and can interact with the stellar mass.
“These extreme events push the boundaries of our understanding of physics, and studying them allows us to learn new things,” Radice said. “The period when merging stars are out of equilibrium is only 2 to 3 milliseconds, but like temperature, time is relative here, the orbital period of two stars before merging 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 explained that the precise physical interactions that occur during the merger can affect the types of signals that can be observed on Earth from merging binary stars.
“How neutrinos interact with the star’s mass and are eventually emitted can affect the oscillations of the merged remnants of the two stars, which in turn can affect what the merger’s electromagnetic and gravitational wave signals look like when they reach us. on Earth,” Espino said. “Next generation gravitational wave detectors could be designed to look for these kinds of differences in the signals. In this way, these simulations play a key role, allowing us to gain insight into these extreme events while informing future experiments and observations in a kind of feedback loop.
Reference: “Neutrino Trapping and Out-of-Equilibrium Effects in Binary Neutron-Star Merger Remnants” by Pedro Luis Espino, Peter Hammond, David Radice, Sebastiano Bernuzzi, Rossella Gamba, Francesco Zappa, Luís Felipe Longo Micchi, and Albino Perego, 20 May 2024, Physical Review Letters.
DOI: 10.1103/PhysRevLett.132.211001
In addition to Espino and Radica, the research team includes postdoctoral researchers Peter Hammond and Rossell Gambo of Penn State; Sebastiano Bernuzzi, Francesco Zappa and Luís Felipe Longo Micchi at the Friedrich-Schiller-Universität Jena in Germany; and Albino Perego at the Università di Trento in Italy.
Funding from the US National Science Foundation; US Department of Energy (DOE), Office of Science, Division of Nuclear Physics; Deutsche Forschungsgemeinschaft; and the European Union’s Horizon 2020 and Europe Horizon initiatives supported this research. The simulations were performed on the Bridges2, Expanse, Frontera and Perlmutter supercomputers. The research utilized the resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility supported by the US Department of Energy’s Office of Science. The authors acknowledge the eV Gauss Center for Supercomputing
for funding this project by providing computing time on the GCS SuperMUC-NG supercomputer at the Leibniz Supercomputing Center.