New simulations reveal hot neutrinos trapped during collisions with neutron stars

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

New simulations of these events show that hot neutrinos (small, essentially massless particles that rarely interact with other matter) created during the collision can become briefly trapped at these interfaces and remain out of equilibrium with the cold cores of the merging stars. 3 milliseconds. During this time, the simulations show that the neutrinos can weakly interact with the stars’ matter, driving the particles back to equilibrium – and providing new insight into the physics of these powerful events.

An article describing the simulations, written by a research team led by Penn State physicists, appeared in the journal Physical Assessment Letters.

“For the first time in 2017, we observed several types of signals here on Earth, including gravitational waves, coming from a merger of binary neutron stars,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley. led the investigation.

‘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 for understanding what happens during a binary neutron star merger is through simulations. based on mathematics derived from Einstein’s general theory of relativity.”

Neutron stars get their name because they are thought to be composed almost entirely of neutrons, the uncharged particles that, together with positively charged protons and negatively charged electrons, form atoms. It is thought that their incredible density – only black holes are smaller and denser – squeezes protons and electrons together, causing them to fuse into neutrons.

A typical neutron star is only tens of kilometers in diameter, but is about one and a half times the mass of our Sun, which is about 1.4 million kilometers in diameter. 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 dense that photons cannot escape to dissipate the heat; instead, we think they cool down by emitting neutrinos.”

According to the researchers, neutrinos are created during the collision because neutrons in the stars collide with each other and are shot 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 as with temperature, time here is relative; the orbital period of the two stars before the merger may be as little as 1 millisecond. The equilibrium phase is the phase where the most interesting physics occurs. Once the system is back in equilibrium, the physics will be 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 could affect the oscillations of the merged remnants of the two stars, which in turn could 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. “

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.