When black holes collide, neutrinos are produced.

Still image from a numerical simulation of an unequal mass binary black hole merger, with parameters consistent with GW190412. Credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project

Astronomers have been unable to determine what causes ultra-high energy neutrinos since they were first spotted coming from random directions in space. However, a recent theory posits an odd source: black hole mergers. Neutrinos are really eerie particles. They have no electric charge and very seldom interact with regular matter via the weak nuclear force. Every second, trillions of neutrinos pass through every square centimeter of your body. As a result, it takes truly massive observatories to capture them.

The IceCube Neutrino Observatory, a set of detectors dug into the Antarctic ice sheet at the South Pole, is the biggest of all. Occasionally, a neutrino may collide with a water ice molecule, causing a flash of light that the observatory can detect. While IceCube has witnessed a plethora of events over the years, a few stand out. Some neutrinos are so energetic that it's difficult to come up with plausible scenarios that could produce them.

Black holes, on the other hand, are possibly the most powerful things in the universe. Their powerful gravity may shatter stars and possibly power the development of jets that can travel tens of thousands of light years into space. So, according to a recent study released on the pre-print service arXiv, black holes may be responsible for the highest energy neutrinos. This, however, cannot be applied to isolated black holes. The black holes must instead be surrounded by an electrically charged plasma. This plasma will create an accretion disk as it swirls around the black hole. In the accretion disk, extremely strong magnetic and electric fields can wind their way around the black hole, causing the material to flow outwards in the form of a jet.

When two black holes merge, the direction of the jet changes and the jets can occasionally get a boost from the gravitational energy released by the merger. According to the authors of the new study, if the conditions are just right, the enhancement of the jet during a merger can power insanely high-energy neutrinos.

To match the measured numbers of high-energy neutrinos recorded by IceCube, the scientists propose that these black holes do not need to combine all that frequently. If neutrinos are driven by supermassive black hole mergers, they only need to collide every 100,000 to 10 million years in each cubic gigaparsec of volume. If the neutrinos are instead driven by stellar mass black hole mergers, they only need to happen 10 to 100 times per year in each cubic gigaparsec of volume.

These are encouraging results since they fall within the predicted range of merger rates for both stellar mass and supermassive black holes. So, in terms of mechanics, it's conceivable. Only more observations will tell, and astronomers will hopefully be able to pinpoint the source of these extremely energetic exotic particles.

Journal Information: Ilja Jaroschewski et al, Extragalactic neutrino emission induced by Supermassive and Stellar Mass Black Hole mergers, arXiv (2022). DOI: 10.48550/arxiv.2210.11337