'Ghost particle' reveals source of mysterious cosmic rays
High-energy neutrino travels 4 billion light-years, then trips Antarctic detector
For more than 100 years, scientists have speculated about the origin of high-energy cosmic rays, a rain of charged particles that bombard Earth from space at close to light speed.
Now a global team of hundreds of researchers says it thinks it has tracked down a source of some of the highest-energy cosmic rays: an unusual galaxy called a blazar about four billion light-years from Earth in the constellation Orion. The findings were unveiled at a news conference Thursday morning.
The researchers found the culprit by using telescopes around the world to follow the trail of a "ghost particle" called a high-energy neutrino that hit a massive underground detector in Antarctica last September.
Such particles "are really a smoking gun for where very high-energy cosmic rays are being produced in the universe," said Darren Grant, a University of Alberta physics professor and the spokesperson for the IceCube Collaboration, which runs the neutrino detector. It includes researchers from 12 countries, including Canada.
It's the first time a high-energy neutrino has been tracked back to its origin, far beyond our own galaxy. That opens the door to a brand-new type of astronomy, the researchers report today in the journal Science — one that uses not just light but also subatomic particles to probe objects in deep space.
"It's a new window into our universe," Grant said.
Ghosts in the accelerator
Neutrinos are tiny, neutral particles nicknamed "ghost particles" because most of the time they pass right through stars, people and other matter without interacting with it. Billions of low-energy neutrinos from the sun pass through one of your thumbnails every second.
But astronomers have long thought some high-energy neutrinos on Earth originate in deep space, in cosmic particle accelerators like exploding stars or the violent conditions that suck matter into supermassive black holes. Such conditions also generate cosmic rays, said Grant. "They come hand in hand."
Those conditions include the supermassive black hole at the centre of a blazar. A blazar is a type of bright galaxy found only far, far from our own Milky Way galaxy. What makes a blazar extra special is that it's positioned in a way that fires jets of light directly at Earth, making it easy to observe that light with telescopes.
More than 2,000 blazars have been found, and scientists have long proposed they might be a source of cosmic rays.
It's impossible to track cosmic rays that hit Earth back to their source because they're charged particles that get pushed and pulled by the web of magnetic fields that criss-cross the universe, taking a meandering path that changes direction many times.
But because high-energy neutrinos don't interact with magnetic fields and only very rarely with matter, they're thought to travel in a straight line from their source to Earth.
Of course, they're just as unlikely to hit Earth as anything else. So researchers tried to maximize their chances of a neutrino hitting their detector by building one that was truly massive. Its sensors are embedded in a cubic kilometre of ice beneath the South Pole.
A cross-section of the detector is about 140 times bigger than the Russian soccer field where Croatia defeated England in the World Cup soccer semifinal on Wednesday. It's more than 2.5 kilometres deep.
And on Sept. 22, 2017, the scientists won the cosmic lottery. A high-energy neutrino that had been travelling at the speed of light straight through galaxies, stars, planets and magnetic fields for four billion years smacked into the centre of an atom in the ice just outside the detector.
The neutrino had an energy 40 times that of the protons produced by the biggest particle accelerator on Earth, the Large Hadron Collider. The impact generated a particle called a muon, which kept going. It plowed through the entire detector for more than a kilometre, leaving behind a straight, diagonal trail, lighting up sensors along the way.
University of Alberta physicist Claudio Kopper did a lot of the analysis that identified it as a high-energy neutrino from deep space and extrapolated where in the sky it had come from, Grant said. As they do around once a month, they sent the co-ordinates to their colleagues at telescopes around the world to see if they could spot where the neutrino came from.
A space telescope called Fermi, operated by NASA, spotted a known blazar in that direction, called TXS 0506+056, that had suddenly started flaring — emitting a lot of light. Fermi looks for gamma ray light, which has an energy similar to that of neutrinos and is thought to come from the same processes.
Fermi sent another alert, and yet more telescopes that specialize in seeing different colours of light and radiation scrambled to have a look. In the end, it was observed by 20 telescopes from the Canary Islands to Chile to Japan, and many of those also saw flaring.
Coincidence?
Of course, this was only one neutrino.
Researchers calculated there's a one-in-1,000 chance that the sight of the neutrino and the blazar flaring at the same time was a coincidence, according to Anna Frankowiak, a researcher at Deutsches Elektronen-Synchrotron (DESY) in Germany, who did a statistical analysis.
So researchers looked at their old data and found more than a dozen other high-energy neutrinos from the blazar's direction during flaring events in 2014 and 2015, before IceCube started sending alerts to astronomers in 2016.
The chance that those events were also coincidences were estimated to be 1 in 5,000.
The fact that chances of coincidences are so small from two independent measurements gives researchers confidence that the blazar is where the neutrino came from, Grant said.
Gregory Sivakoff, a University of Alberta researcher, and his team had to write an emergency proposal and get it approved by peer reviewers in just days so they could study the blazar using the Very Large Array radio telescope in New Mexico.
He says this is the first step in proving blazars are a source of cosmic rays, but there is lots of work to do. For one thing, astronomers don't like to base all their conclusions on one source. And even if blazars are confirmed to be a source of cosmic rays on Earth, researchers want to know what fraction of the rays they produce.
More importantly, he said, the study opens up new avenues — effectively new senses — for observing the universe, the way we can enjoy a meal by seeing, smelling, tasting and feeling it. For a long time, astronomers were limited to observing space with the light detected using telescopes. But that's changed with the recent ability to track gravitational waves — and now neutrinos — to objects in space.
"We're entering an age where we're able to use more than one sense at the same time, and I think it will lead to a much greater understanding of how the universe works."
This isn't the first time neutrinos from outside the solar system have been linked to an event in space. A burst of lower-energy neutrinos was detected by three detectors on Earth during a supernova in 1987 in one of the Milky Way's nearby "satellite" galaxies.
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