- The origin of the universe’s most powerful cosmic rays, or high-speed particles, has for decades been difficult to determine.
- Astronomers recently used ghostly particles called neutrinos to verify the source of high-energy cosmic rays.
- IceCube, a huge detector embedded in the ice of Antarctica, led to the discovery.
- Enormous, rapidly spinning black holes called blazars appear to emit cosmic rays.
Astronomers say they’ve more or less confirmed a key source of cosmic rays – some of the highest-energy yet most enigmatic radiation in the universe – with the detection of a single “ghostly” particle in Antarctica.
Cosmic rays were discovered more than 100 years ago, but their origins are tough to know for sure because they can be deflected en route to Earth, and our planet’s atmosphere absorbs most of them.
Researchers detected the “ghost” particle, or neutrino, in September using IceCube, a huge array of sensors embedded deep in the ice of Antarctica. The neutrino was unusually energetic, and when scientists tracked the particle back to its source they found a galactic monster called a blazar: a rapidly spinning black hole, millions of times the mass of the sun, that’s gobbling up gas and dust.
The blazar is called TXS 0506+056, and it appears in the sky just below the arm of the constellation Orion. The supermassive black hole is located 4 billion light-years from Earth, in the core of what’s called an active galaxy.
After the high-energy neutrino’s detection, IceCube alerted other astronomers, who aimed a suite of light-based observatories at the blazar. Those telescopes and detectors captured a massive burst of other radiation.
“This is the first evidence that we have of an active galaxy emitting neutrinos, which means we may soon start observing the universe using neutrinos to learn more about these objects in ways that would be impossible with light alone,” Marcos Santander, an astrophysicist at the University of Alabama, said in a press release.
Santander and dozens of others have detailed their research in two studies published Thursday in the journal Science. The first study links the high-energy neutrino – which researchers believe was generated by cosmic rays – to the blazar. The second shores up more evidence for the link by finding many more lower-energy neutrinos apparently emanating from the blazar.
Erin Bonning, an astrophysicist at Emory University who studied blazars and wasn’t involved in the research, said the work – if the statistical details bear out – is “really cool” because it’s only the second time that a high-energy neutrino has been linked to source outside our solar system. The first was a couple dozen neutrinos emitted by a supernova in 1987.
Blazars represent a potentially large source of cosmic rays, since there are hundreds of billions of galaxies in the visible universe and at any time about 10% are “active,” meaning they’re eating matter. Feeding black holes shoot out jets of high-energy radiation and particles at close to the speed of light, and most of the time they’re not pointed at Earth.
“The thing that makes a blazar different from active galactic nuclei is that a blazar is when Earth is looking down the barrel of a jet,” Bonning said.
The grand mystery – and importance – of cosmic rays
- Sophia Dagnello, NRAO/AUI/NSF
Cosmic rays constantly bombard the Earth. Also called cosmic ionizing radiation, the particles are the cores of atoms or subatomic pieces of their cores, called protons, moving at nearly the speed of light. They can travel for billions of years through space before randomly hitting Earth.
You can’t see these high-energy charged particles, but at any given moment tens of thousands of them are soaring through space and slamming into Earth’s atmosphere from all directions.
These rays don’t pose much risk to humans on Earth’s surface, since the planet’s atmosphere and magnetic field shield us from most of the threat.
“Cosmic rays are not a significant exposure risk on the ground,” Eddie Semones, a radiation health officer at NASA, previously told Business Insider. “You actually get more exposure from the Earth’s natural radioactive material than from galactic cosmic rays.”
Many cosmic rays appear to come from exploding stars called supernovas, though their exact contribution to the total amount of the radiation, or flux, is not certain. The two new studies found that blazars appear to be an important source of cosmic rays – some of the highest-energy ones around.
Bonning said that making that connection is important, though not unexpected.
“The physics behind blazars producing cosmic rays is not controversial,” she said. “It’s generally accepted these are the types of sources that can produce cosmic rays.”
But researchers have struggled for so long to make such links because all the stuff in the universe – especially magnetic objects, gas, and dust – can mess with the path of cosmic rays on their journey to Earth.
“You can see a cosmic ray coming from over there, but that doesn’t necessarily mean it came from that way,” Bonning said. “They can spiral around.”
That’s where neutrinos come in. Trillions from the sun pass through the palm of your hand in one second, since they ignore nearly all other stuff in the universe.
How astrophysicists linked one neutrino to a major discovery
IceCube does not detect neutrinos directly but instead records their rare interactions with molecules. Every now and then, after trillions and trillions of neutrinos pass by uninhibited, one will occasionally whack into a large particle and propel it faster than the speed of light.
This is possible because light travels more slowly through ice. However, when that universal speed limit is broken by particles of matter, they shed a bunch of photons (particles of light) in response.
An array of photon detectors in ice can thus help scientists rebuild the path of a neutrino by looking at sequences of how they were set off.
Bonning said researchers outside the IceCube Collaboration, and other teams involved in the work, would most likely question the strength of the connection because of the statistics: Both studies are based on the detection of a single neutrino.
“The probability of this being chance is 1 in 1,000, or what scientists call three-sigma,” she said, adding: “This is the bare minimum to claim you’ve detected something.”
But Bonning said the second study bolstered the work, since it compares previous observations of blazar TXS 0506+056 with lower-energy neutrinos detected by IceCube. That study found the same blazar to be the source with a better than 1-in-1,000 chance.
Bonning said that beyond possibly confirming blazars as a source of high-energy cosmic rays, the studies strengthen the possibility of using neutrinos as a kind of telescope for objects beyond our solar system. That’s important because we still know so little about distant galaxies, let alone cosmic rays.
“Like the detection of gravitational waves, it’s the beginning of some new discoveries,” she said. “Over time, they’ll be able to correlate other neutrinos with similar objects.”