Every so often, protons and even entire atomic nuclei strike the Earth with extremely high energies—much higher than what scientists can produce in their most powerful physics experiments. Since the discovery of “cosmic rays” a hundred years ago, no one knew for sure where the most energetic of these particles came from—until now.

On Thursday, scientists from institutions across the globe announced they have confirmed a source of these ultra-high-energy cosmic rays, thanks to combined data from light and a single high-energy neutrino particle. That source is a blazar, a supermassive black hole at the center of a galaxy emitting high-energy jets of light and radiation pointed straight at Earth. The discovery is a highlight of the new “multimessenger” era of astronomy, in which particles other than light, like neutrinos, aid in scientists’ understanding of the cosmos.

“After a century, we finally found a source of the cosmic rays,” Francis Halzen, University of Wisconsin physicist and principle investigator of the IceCube Neutrino Observatory, told Gizmodo. “But what’s equally as exciting is the way we found it.”

On September 22, 2017, the IceCube detector at the South Pole spotted a particle called a neutrino with an incredible amount of energy—290 tera-electron volts, or more than 290,000 times the energy of proton at rest, and tens of times more energetic than the protons at the Large Hadron Collider.

The detector’s computers calculated where the neutrino came from, and sent the coordinates in a message to astronomers around the world. Six days later, the Earth-orbiting Fermi Large Area Telescope reported that a blazar called TXS 0506+056, four billion light-years away, was in the same spot, according to the paper published today in Science. A bright black hole at the center of a galaxy was eating up the dust surrounding it and spitting out high-energy jets of particles and light in our direction. Other telescopes continued looking to get more detail.

We sensed this neutrino, then Fermi told us there was a blazar right in the path of the neutrinos, then all of the telescopes started looking,” said Halzen. “If there hadn’t been a multimessenger campaign, this source would never have been identified as something special.”

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The discovery was a long time coming. The story goes that in 1912, Austrian physicist Victor Hess realized that there’s more radiation higher up in the atmosphere, where particles slam the Earth from outer space. Sure, we get radiation from the Sun and cosmic rays from supernovae, but physicists couldn’t figure out what sent the highest-energy particles, the protons and entire atomic nuclei. These could have energies higher than 1020 electron volts. Which, well, that’s a lot.

More recently, scientists observed the coincidence of neutrinos and light from the well-studied supernova of 1987, explained Freya Blekman, a physicist at the Vrije Universiteit Brussel who was not involved with the study. Ever since then, “physicists have been trying to test if it is indeed true that astrophysical objects are producing signatures as, for example, cosmic ray particles, optical light, radio, and neutrinos.”

And since the IceCube detector at the South Pole fully came online in 2010, it has measured extremely high-energy neutrinos without an obvious source.

A schematic of the event in IceCube’s strings
Graphic: IceCube/NSF

Today’s announcement ties all of these loose ends together. At IceCube, 86 strings are buried under a mile of ice, with over 5,000 light sensors covering a cube a kilometer on a side. Neutrinos barely interact with matter on Earth, but when they have enough energy, they can interact with the ice near the detector and produce a flash from resulting particles traveling faster than the ice would normally allow.

The neutrino pointing right back toward the blazar is most likely not a coincidence. “Thanks to the Fermi satellite, we found three sigma for it to be chance,” Elisa Bernardini, professor for Gamma-ray and Neutrino Astroparticle Physics at the Humboldt University of Berlin, explained to Gizmodo. “That means it leaves very little room for saying the neutrinos aren’t correlated with this particular object.” Physicists have since looked at almost a decade of past IceCube data, and found even more high-energy neutrino events coming from TXS 0506+056's direction in 2014 to 2015, according to a second paper published today in Science. 

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That explains the neutrinos, but how does it explain the ultra-high-energy cosmic rays? Cosmic ray origins have remained a mystery because they don’t travel in straight lines; they’re bent by the magnetic fields permeating the intervening space. That means if you look at the direction from which they came, it probably won’t be aligned with whatever spat them out—this specific blazar’s cosmic rays probably don’t hit the Earth, for example. Neutrinos and photons don’t bend, though. High-energy photons aren’t necessarily indicative of cosmic rays, and can come from electrons, said Bernardini. But high-energy neutrinos would only appear from interactions with hadrons, things made from quarks like protons or even entire atomic nuclei. Things like cosmic rays.

So, not only do blazars produce high-energy gamma rays, but they also produce high-energy protons and nuclei, which then produce neutrinos. In other words, blazars can produce the ultra-high-energy cosmic rays that have perplexed scientists for decades.

This discovery certainly wasn’t easy, said Blekman. “You have no choice of experiments in astronomy, you just have to be lucky that something ‘flies’ into your telescope or that your telescope was pointing the right way when something extreme happens.”

And while this may be an exciting observation, it’s just one, so it can’t explain all of the cosmic rays or how they’re made. “We clearly need more data. One source is not enough,” physicist Spencer Klein at Lawrence Berkeley National Lab told Gizmodo. “Now that we found one accelerator, we’d like to find more and find out how they work.”

All of this work just goes to show the importance of having lots of telescopes working together observing the skies. “Had we not sensed the neutrino, this would not have happened,” said Halzen. “Any single telescope could not have made this breakthrough.”

[Science, Science]