Simulation of the last moments before the merger of neutron stars

When stars reach the end of their life cycle, they shed their outer layers in a supernova. What is left behind is a neutron star, a stellar remnant that is incredibly dense despite being relatively small and cool. When this happens in binary systems, the resulting neutron stars will eventually spin in and collide. When they finally merge, the process causes gravitational waves to be released and can lead to the formation of a black hole. But what happens when neutron stars start merging, down to the quantum level, is something scientists are eager to learn more about.

When stars begin to merge, very high temperatures are generated, creating “hot neutrinos” that remain out of equilibrium with the cool cores of the merging stars. Usually, these tiny, massless particles interact with normal matter only through weak nuclear forces and possibly gravity. However, according to new simulations led by Penn State University (PSU) physicists, these neutrinos may interact weakly with normal matter during this time. These findings may lead to new insights into these powerful events.

Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, led the research. He was joined by astrophysicists from PSU, the Institute for Theoretical Physics at Friedrich Schiller University Jena, the University of Trento and the Trento Institute for Fundamental Physics and Applications (INFN-TIFPA). A paper describing their simulations, “Neutrino trapping and out-of-equilibrium effects in binary remnants of neutron star mergers,” recently appeared in the journal Physical review papers.

Originally predicted by Einstein’s Theory of General Relativity, gravitational waves (GWs) are essentially ripples in spacetime caused by the collapse of stars or the merger of compact objects (such as neutron stars and black holes). Neutron stars are so named because their incredible density fuses protons and electrons together, creating stellar debris composed almost entirely of neutrons. For years, astronomers have studied GW events to learn more about binary companions and what happens when they merge. Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, explained in a Penn State news release:

“For the first time in 2017, we observed here on Earth signals of various kinds, including gravitational waves, from a binary neutron star merger. This led to a great increase of 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 into understanding what happens during a binary neutron star merger is through mathematically based simulations arising from Einstein’s theory of general relativity.

While neutron stars are effectively cold, they can become extremely hot during a merger, especially at the interface (the point where the two stars are making contact). In this region, temperatures can reach trillions of degrees Kelvin, but the density of stars prevents photons from escaping to dissipate the heat. According to David Radice, an assistant professor of astronomy and astrophysics in Penn State’s Eberly College of Science and one of the team’s leaders, this heat may be dissipated by neutrinos, which are created during the collision as neutrons break up to form protons. electrons and neutrinos.

“The period when 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 little as a millisecond,” he said. “This brief out-of-equilibrium phase is when the most interesting physics happens. Once the system returns to equilibrium, the physics is better understood.”

To investigate this, the research team created supercomputer simulations that modeled the merger and associated physics of binary neutron stars. Their simulations showed that neutrinos can also be trapped by the heat and density of the merger, that hot neutrinos are out of equilibrium with the still-cool cores and can interact with stellar matter. Moreover, their simulations show that the physical conditions present during a merger can affect the resulting GW signals. Espino said:

“How neutrinos interact with the star’s matter and are eventually emitted can affect the oscillations of the merging 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. here. on the ground. Next generation gravitational wave detectors can be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role in allowing us to gain insight into these extreme events informing future experiments and observations in a kind of feedback loop.”

This is certainly good news for gravitational wave astronomy and for scientists hoping to use GW events to probe the interior of neutron stars. Knowing the conditions present during mergers based on the type of GW signals produced may also provide new insight into supernovae, gamma-ray bursts, radio bursts, and the nature of dark matter.

Further reading: PSU, Physical review papers