Neutrinos are some of the weirdest and most mysterious particles in the universe, but we are starting to learn more about their origins. For the first time, a team of researchers has traced a single high-energy neutrino back to its birthplace, a supermassive black hole around 4 billion light years away.
On 22 September last year, a blue light pulsed through the ice deep under the South Pole. This light was generated by a high-energy neutrino passing through Earth and picked up by an experiment called the IceCube detector that traced the path of the light  back to the direction the neutrino came from.
Less than a minute later, IceCube sent an automated alert to astronomers around the world, telling them to turn their telescopes on a small area in the sky. They scrambled to look for anything bright that might be generating high-energy protons, which interact with other matter and light to create neutrinos.
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A few days later, observers using NASA’s Fermi space telescope reported that the neutrino’s path pointed back in the direction of a blazar – a supermassive black hole blasting out a jet of particles – that was experiencing a huge flare, and likely producing neutrinos.
Spectacular first
“This is a pretty spectacular first,” says IceCube spokesperson Darren Grant. “It’s the first compelling evidence of a high-energy neutrino being measured from where it originated.” And it’s only the third time we’ve found a specific cosmic object creating neutrinos, the other two being the sun and a nearby supernova in 1987.
When IceCube researchers went back through almost a decade of data, they found 13 other neutrinos that seem to have come from the same direction in 2014 and 2015. They came in before the alert system was online, and wouldn’t have been quite high-energy enough to trigger an alert anyway.
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“It doesn’t seem like much, but prior to this the best source that we’ve ever looked at had three neutrinos coming from it,” says IceCube team member Josh Woods.
With Fermi pin-pointing the source, other telescopes turned to look at the blazar, taking observations in wavelengths across the electromagnetic spectrum. It’s extremely bright in many wavelengths, making it able to accelerate protons to the extremely high energies required to create September’s neutrino. Researchers also measured its distance, which wasn’t precisely known before, at about 4 billion light years away.
Mystery solved
The find solves a long-lasting mystery, one that’s been around since 1912, when astronomers first spotted another kind of high-energy particle from space called cosmic rays. Unlike neutrinos, cosmic rays interact with magnetic fields as they travel, giving them twisting and turning paths. That means we’ve never definitively pinpointed a source of cosmic rays.
But cosmic rays are mostly protons – the same protons that create high-energy neutrinos – so the blazar that produced the neutrinos must also be producing some high-energy cosmic rays.
Blazars can’t be responsible for producing all of the high-energy neutrinos and cosmic rays that we see, simply because there aren’t enough of them. Some must come from other violent environments and processes in the sky such as supernovae and gamma ray bursts, but having such detailed observations of one source may help us narrow down exactly how they’re produced.
Fitting everything into one coherent picture will be very challenging, says Brad Cenko at NASA’s Goddard Space Flight Center in Maryland. The processes that create both neutrinos and light across the electromagnetic spectrum seem more complicated than we predicted, and we are not even completely sure what they are yet, he says.
But as neutrino detectors continue to improve, we are likely to find many more sources with more high-energy neutrinos associated with them. Then, we’ll be able to use neutrinos to probe how these cosmic particle accelerators work.
“The idea that neutrino astronomy works has really come to reality now,” says Grant. “For the longest time, people studied the cosmos through the electromagnetic spectrum, and recently we added gravitational waves which was just an incredible breakthrough, and now neutrinos join that party.”
Journal reference:ĚýScience,Ěý;Ěý
