Born in the cradle of deep space, blasting through the universe at nearly the speed of light and harnessing energy up to a million times greater than anything achieved by the most powerful particle accelerator world, cosmic rays are fragments of atoms that rain down relentlessly on Earth. They get caught in our atmosphere and disrupt our satellites. They threaten the health of astronauts living in orbit, even when there are few of them.
What kind of extreme cosmic factory could manufacture such a thing, you ask? Not clear. In fact, this question has plagued scientists for more than a century. But on Thursday in the journal Science, astrophysicists announced they may have discovered an important clue to piecing together a cosmic ray origin story.
The short version is that they think cosmic rays come from blazars or galaxies containing huge black holes with energetic jets pointing towards Earth – streams so intense they are even more powerful than the entire galactic region. surrounding. These are the kinds of phenomena that one would expect ferocious particles to come from.
“That, of course, means we’re sitting right in the particle beam being spit out at us by the black hole,” Francis Halzen, professor of physics at the University of Wisconsin-Madison and lead scientist at the IceCube Neutrino Observatory, who was not involved in the new study, said in a statement.
Here is the long version.
A secret neutrino code
Basically, the team in the new study used the art of deduction to work out where these strange bits of atoms came from.
First, they spotted a kind of cosmic ray offspring called a neutrino.
Known as “ghost particles”, neutrinos are a huge enigma in themselves. They are so evasive that they interact with almost nothing, while blasting with force throughout the cosmos. During their journey, neutrinos do not touch even the smallest building blocks of life – atoms – which means that billions of them pass through your atoms right now. You can’t tell.
Specifically to cosmic rays, however, neutrinos are thought to start somewhere along the lifetime of puzzling particles. Their legacies are linked, so to speak.
Thus, the research team realized that if we can understand where astrophysical neutrinos come from, we will also have a clear idea of the origin of cosmic rays. Think of neutrinos as little dark messengers, telling us where their parent cosmic rays are. Fascinatingly, these sorts of “particle messengers” are giving rise to a whole new field of astronomy called multi-messenger astronomy.
Rather than relying solely on light to decode the universe – the driving force behind NASA’s exceptional James Webb Space Telescope, for example – scientists can call on elusive particles, and even gravitational waves, to dissect the ins and outs of spatial phenomena. .
“It’s like feeling, hearing and seeing at the same time. You’ll get a much better understanding,” Marco Ajello, associate professor of physics and astronomy at Clemson University and study author, said in a statement. . “The same is true in astrophysics, because the information you get from multiple detections of different messengers is much more detailed than what you can get from light alone.”
Search from the South Pole
So, focusing on multi-messenger astronomy, to get to the bottom of it, the scientists first analyzed what they call the “largest available neutrino data set” optimized for research, collected from the IceCube Neutrino Observatory, a science base buried deep within. the south pole. In 2017,.
But there was still some debate over whether these blazars are really natural particle accelerators that make cosmic rays. Other experts, for example, believe that cosmic rays are bursts of stardust crashing through space, the product of violent supernovae illuminating the universe.
While that debate should be closed, according to the new study team, because they cross-checked the IceCube findings with a catalog of blazars – the PeVatron blazars, to be exact, which accelerate particles to at least 10^15 electron-volts — and got strong evidence that the two are entangled.
“In this work,” the study authors wrote, “we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at an unprecedented level of confidence.”
“We had a clue then (in 2017), and now we have proof,” Ajello said.
“The results provide, for the first time, compelling observational evidence that the subsample of PeVatron blazars are extragalactic neutrino sources and thus cosmic ray accelerators,” said study co-author Sara Buson of the Julius-Maximilians-Universität in Germany. statement.
Importantly, Buson also notes that these results come from looking at the “most promising” sets of IceCube neutrino data – meaning that digging deeper into the background sets could offer even stronger evidence. and pave the way for further discoveries in the future.
As Aljello puts it, this new neutrino clue “takes us one step closer to solving the century-old mystery of the origin of cosmic rays.”