During the collision of binary neutron stars, hot neutrinos can become briefly stuck at the interface, leaving them out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. This interaction helps drive the particles toward equilibrium and provides new insights into the physics of such mergers. Credit: SciTechDaily.com
New simulations show that neutrinos were created during these catastrophic events. neutron star collisions are briefly thrown out of thermodynamic equilibrium with the cold cores of the merging stars.
Recent simulations by Penn State physicists have shown that hot neutrinos can briefly become trapped and out of equilibrium when binary neutron stars merge, providing new insight into 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 cold remnants called neutron stars. When two stars collapse close to each other, the remaining binary neutron stars spiral and eventually collide, heating the collision point to extreme temperatures.
New simulations of these events show that hot neutrinos — small, essentially massless particles that rarely interact with other matter — created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold nuclei for 2 to 3 milliseconds of the merging stars. During this time, the simulations show that the neutrinos can interact weakly with the stars’ matter, helping to drive the particles back to equilibrium — and offering new insights into the physics of these powerful events.
Groundbreaking simulations of neutron star mergers
A paper was recently published in the journal describing simulations by a research team led by Penn State physicists. Physical Assessment Letters.
“For the first time in 2017, we observed signals of various kinds here on Earth, including gravitational waves“from a merger of two neutron stars,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, who led the research. “This led to a huge increase in interest in the astrophysics of binary neutron stars. There is no way to reproduce these events in a laboratory to study them experimentally, so the best window we have to understand what happens during binary neutron star mergers is through simulations based on mathematics that comes from Einstein’s general theory of relativity.”
Volume rendering of density in a simulation of a binary neutron star merger. New research shows that neutrinos created in the hot interface between merging stars can be briefly captured and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. Credit: David Radice, Penn State
Composition of neutron stars and collision dynamics
Neutron stars get their name because they are thought to be made almost entirely of neutrons, the uncharged particles that, together 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 the 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.
“Neutron stars are actually cold before merger, 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 very hot; the interface of the colliding stars can be heated to temperatures in the trillions of degrees Kelvin. However, they are so compact that photons cannot escape to dissipate the heat; instead, we think they cool down by emitting neutrinos.”
Insights from the behavior of neutrinos in stellar mergers
According to the researchers, neutrinos are created during the collision when neutrons in the stars collide and are blown apart into protons, electrons and neutrinos. What happens in those first moments after a collision is an open question in astrophysics.
To answer that question, the research team created simulations that require enormous amounts of computing power and model the merger of binary neutron stars and all the physics involved. The simulations showed for the first time that even neutrinos, no matter how briefly, can be trapped by the heat and density of the fusion. The hot neutrinos are not in equilibrium with the still cool cores of the stars and can interact with the matter of the stars.
“These extreme events push the boundaries of our understanding of physics and by studying them we can learn new things,” Radice said. “The period during which 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 merger can be as short as 1 millisecond. This short phase of out-of-equilibrium equilibrium 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 take place during the merger could influence the types of signals that can be observed on Earth from binary star mergers.
“How the neutrinos interact with the stars’ matter and are ultimately emitted can affect the oscillations of the merged remnants of the two stars, which in turn can affect what the electromagnetic and gravitational wave signals from the merger look like as they reach us here. on Earth,” Espino said. “Next-generation gravitational wave detectors could be designed to look for these types of signal differences. In this way, these simulations play a crucial role allowing us to gain insight into these extreme events while simultaneously 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, May 20 2024, Physical assessment letters.
DOI: 10.1103/PhysRevLett.132.211001
In addition to Espino and Radice, the research team includes postdoctoral scientists Peter Hammond and Rossella Gamba from 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 U.S. National Science Foundation; the U.S. Department of Energy (DOE), Office of Science, Division of Nuclear Physics; the Deutsche Forschungsgemeinschaft; and the European Union’s Horizon 2020 and Europe Horizon initiatives supported this research. Simulations were performed on Bridges2, Expanse, Frontera, and Perlmutter supercomputers. The research used resources from the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the U.S. Department of Energy’s Office of Science. The authors acknowledge the Gauss Centre for Supercomputing e.V.
to fund this project by providing computing time on the GCS Supercomputer SuperMUC-NG at the Leibniz Supercomputing Center.