When black holes collide, they also produce neutrinos

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

Neutrinos are extremely ghostly particles. They carry no electrical charge and only rarely interact with normal matter through the weak nuclear force. Billions of neutrinos pass through every square inch of your body every second. So you need really huge observatories to capture them.

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

Although IceCube has seen countless events over the years, a few stand out. Some neutrinos are extremely energetic, so energetic that it is difficult to come up with plausible scenarios that could generate 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 propel tens of thousands of light-years into space.

Thus, new research, published on the preprint server arXiv, suggests that black holes could be responsible for the most energetic neutrinos. However, this cannot work with isolated black holes. Instead, black holes must be surrounded by an electrically charged plasma. This plasma will swirl around the black hole forming an accretion disk. Incredibly strong magnetic and electric fields in the accretion disk can wrap around the black hole and send matter outward in the form of a jet.

When two black holes merge, it changes the direction of the jet, and sometimes 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 ideal, the jet enhancement during a merger can fuel incredibly energetic neutrinos.

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

These are promising numbers because the results fall within the expected range of fusion rates for stellar-mass black holes and supermassive black holes. As the mechanics go, it’s plausible. Only more observations will be able to tell, and hopefully astronomers will be able to identify a source of these extremely energetic exotic particles.