New simulations reveal hot neutrinos trapped during neutron star collisions

When stars collapse, they can leave behind incredibly dense but relatively small and cold remnants called neutron stars. If two stars collapse in close proximity, the leftover binary neutron stars spiral in and eventually collide, and the interface where the two stars begin merging becomes incredibly hot.

New simulations of these events show hot neutrinos—tiny, essentially massless particles that rarely interact with other matter—that are created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. During this time, the simulations show that the neutrinos can weakly interact with the matter of the stars, helping to drive the particles back toward equilibrium—and lending new insight into the physics of these powerful events.

A paper describing the simulations, by a research team led by Penn State physicists, appeared in the journal Physical Reviews Letters.

"For the first time in 2017, we observed here on Earth signals of various kinds, including gravitational waves, from a binary neutron star merger," 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 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."

Neutron stars get their name because they are thought to be composed almost entirely out of neutrons, the 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 might weigh as much as a mountain, tens or hundreds of millions of tons.

"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," said David Radice, assistant professor of physics and of astronomy and astrophysics in the Eberly College of Science at Penn State and a leader of the research team.

"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."

According to the researchers, neutrinos are created during the collision as neutrons in the stars smash into each other and are blasted apart into protons, electrons and neutrinos. What then happens in those first moments after a collision has been an open question in astrophysics.