When black holes collide they also produce neutrinos

Ever since astronomers first spotted extremely high-energy neutrinos coming from random directions in space, they haven’t been able to figure out what creates them. But a new hypothesis suggests an unlikely source: black hole mergers.

Neutrinos are extremely eerie particles. They carry no electric charge and interact only rarely with normal matter through the weak nuclear force. Trillions of neutrinos pass through every square centimeter of your body every second. So really huge observatories are needed to capture them.

The largest of all is the IceCube Neutrino Observatory, which is a series of detectors submerged in the Antarctic ice sheet at the South Pole. Occasionally a neutrino will hit a water ice molecule and lead to a flash of light that the observatory can detect.

While IceCube has seen countless events over the years, a few stand out. Some neutrinos are extremely energetic — so energetic that it is difficult to find plausible scenarios that could produce them.

At the other end of the spectrum, perhaps the most powerful objects in the universe are black holes. Their intense gravity can tear apart stars and even fuel the formation of jets that can shoot tens of thousands of light years into space.

So new research, published on the arXiv preprint server, suggests that black holes may be responsible for higher-energy neutrinos. However, this cannot work with black holes in isolation. Instead, black holes must be surrounded by an electrically charged plasma. This creature will swirl around the black hole forming an accretion disk. The incredibly strong magnetic and electric fields in the accretion disk can wrap around the black hole and send material streaming out in the form of a jet.

When two black holes merge, this changes the direction of the jet, and occasionally the jets can be boosted by the gravitational energy released by the merger.

The authors of the new study suggest that if the conditions are just right, the jet boost during a merger can fuel insanely high-energy neutrinos.

To match the observed number of high-energy neutrinos detected by IceCube, the authors suggest that these black holes do not need to merge as often. If neutrinos are powered by supermassive black hole mergers, then they only need to collide between every 100,000 and 10 million years per cubic gigaparsec of volume. If instead neutrinos are powered by stellar-mass black hole mergers, then they need only occur 10 to 100 times each year in each cubic gigaparsec of volume.

These are promising numbers because the results are within the expected range of merger rates of both stellar mass black holes and supermassive black holes. As mechanics go, it makes sense. Only more observations will be able to tell, and hopefully astronomers will be able to identify a source of these highly energetic exotic particles.