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IceCube is the largest neutrino observatory in the world
Photo Credit: Benjamin Eberhardt
IceCube is the largest neutrino observatory in the world and consists of over five thousand optical detectors draped through a cubic kilometer of ice at the geographic South Pole. IceCube was built specifically to study cosmic neutrinos that come from outside our own solar system.

Where are IceCube’s neutrinos coming from?

Scientists define the most likely sources of cosmic neutrinos to hit detectors at the South Pole

Thanks to the IceCube Neutrino Observatory, scientists have identified several types of cosmic structures that produce neutrinos. A new study estimates for the first time how likely a neutrino is to come from each source type, helping physicists understand more about these ghostly particles and how they are created in the universe.

Neutrinos are tiny, nearly massless elementary particles that travel through the universe at almost the speed of light. They were first made during the Big Bang and are produced today by fusion reactions inside stars (including our own Sun), by supernovae explosions when massive stars die, and by the violent transformations of matter and energy that happen around black holes.

The IceCube Neutrino Observatory, or IceCube as it’s commonly called, has detected over 100 cosmic neutrinos since its first full science run in 2011, and scientists have figured out where a few of them came from: a flaring blazar, a nearby Seyfert galaxy, and a tidal disruption event, the term for when a star wanders too close to a supermassive black hole and gets gobbled up by it.

But neutrinos don’t come from all of these sources equally. In the new study, physicists used statistics to determine what percentage each of these sources contributes to the total flux of cosmic neutrinos IceCube detects. It’s a complicated calculation that depends on how many of these structures there are in the known universe, how often they can produce a neutrino, and how likely that neutrino is to reach Earth.

The study authors created a pie chart, illustrated below, that shows the relative contribution of each neutrino source to the total flux of cosmic neutrinos hitting IceCube’s sensors.

The IceCube pie chart shows what astronomical structures and events are creating neutrinos.
Graphic by Lauren Lipuma
The IceCube pie chart shows what astronomical structures and events are creating neutrinos.

Blazars account for an only small fraction of the total neutrino flux, so it’s surprising that IceCube has already detected one from these cosmic accelerators, said Imre Bartos, a physicist at the University of Florida and lead author of the new study.

“It was surprising to some at least that the first detection of a neutrino from an astrophysical source that we see was actually a blazar,” he said.

It’s because blazars are so rare, Bartos explained. There are so few blazars compared to other types of active galactic nuclei that they’re easy for astronomers to identify, which makes it easier to associate one with a neutrino.

Tidal disruption events are rare and provide little matter to black holes compared to the amount of gas and dust they continuously consume as active galactic nuclei, so Bartos was surprised they make up such a large fraction of the total cosmic neutrino flux.

One interesting finding was that other unknown sources make up such a large fraction of the cosmic neutrinos reaching Earth.

“We can see this indication of an extra source type or multiple source types that are not within these three,” Bartos said. “So that was the other major surprise we found in the pie chart.”

One question is whether these unknown neutrino sources are connected to supermassive black holes the way active galactic nuclei, blazars, and tidal disruption events are. Could neutrinos be coming from smaller black holes, supernovae, or other unsuspected cosmic objects?

“We have now at least four source types, which means that we have a very good chance of learning much more about the Universe by looking at them and studying these more,” Bartos said. “So this is really, really exciting.”

And the more neutrino sources that IceCube can identify, the more scientists can learn about the cosmos and the chaotic environments that create neutrinos.

“The key goal is to use these sources to dig deeper and understand what is actually accelerating these particles to the highest energies a billion times higher than what we can do here on Earth,” Bartos said.

IceCube is operated by the University of Wisconsin-Madison and supported by the National Science Foundation (Award No. 2042807), which manages the U.S. Antarctic Program. NSF-funded research in this story: Imre Bartos, University of Florida, Award No. 1911796