New simulations reveal hot neutrinos captured in neutron star collisions

When stars collapse, they can 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, and the interface where the two stars begin to merge becomes incredibly 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 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.

An article describing simulations by a research team led by Penn State physicists appeared in the journal Physical review letters.

“For the first time in 2017, here on Earth, we observed signals of various kinds, including gravitational waves, from binary neutron star mergers,” said Pedro Luis Espino, a postdoctoral fellow at Penn State and the 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 and 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 derives from Einstein’s theory of general relativity.”

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, even though 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. But they’re so dense that photons can’t escape to dissipate the heat; instead, we think they cool down by emitting neutrinos.”

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. -the 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 stellar mass and are eventually emitted can affect the oscillations of the merged remnants of 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 through feedback.” loop.”

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.