When a galaxy gets pulled into the supermassive black hole at its center — yes, that happens — it can create a blazar — an intense beam of high-energy radiation.
There are thousands of blazars across the universe, but on Thursday, a team of physicists, working with a lab near the South Pole, said they’ve discovered that blazars can also double as particle accelerators.
They report, in two papers published in the journal Science, that blazars are a source of high-energy neutrinos — elementary particles of the universe that could help trace the origin of cosmic rays.
The universe is full of neutrinos, but we barely know that they are there. Trillions of these elusive particles pass through your body every second, stopped only by a head-on collision with other particles, like the nucleus of an atom. In 1998, researchers discovered that neutrinos have mass, upending what was thought about how these ghost particles and possibly breaking the Standard Model of physics.
This latest discovery is important because, after the sun and supernovas, blazars are only the third known source of neutrinos, and learning about them could help with a number of missions, from understanding the foundations of the universe to detecting illegal nuclear weapon development.
What the scientists did:
Neutrinos are hard to detect. Over the course of seven years, a team of researchers built the IceCube Neutrino Observatory, a cube of Antarctic ice more than half a mile on each side, with the goal of spotting rare high-energy neutrinos.
The project includes 300 scientists in 12 countries across four continents. More than 5,000 detectors are buried deep in the ice — some more than a mile down. The holes for the detectors were “drilled” into the ice with hot water. Once the detectors reached the appropriate depth, the water froze them in place.
Here’s how it works: When a neutrino hits an atom’s nucleus within the ice, it starts a chain reaction that eventually creates a cone of blue light.
When this light hits the detectors around it, an electrical signal is sent from the detector to a computer at the surface. Scientists convert these “snapshots” of the neutrinos into a 3D movie that shows exactly where the energetic particle passed through IceCube. Even at this advanced observatory, spotting neutrinos is rare. But when celestial objects “flare,” they spit out many more neutrinos than usual, and the chances go up.
What the scientists found:
Last fall, one neutrino set off a different kind of chain reaction. Within a minute of the neutrino’s detection, IceCube notified more than 20 observatories across the world, which then subsequently pointed their telescopes in the direction of the neutrino’s origin.
At the MAGIC telescope in Spain, scientists saw a large number of high-energy gamma rays coming from that direction. So did scientists at NASA’s Fermi space telescope, which had been tracking high-energy gamma rays from a blazar in this direction for 10 years.
Together, they confirmed that this blazar had “flared” — giving off much more energy than usual — right before the neutrino was detected on Earth. Fermilab and IceCube confirmed that this blazar had produced neutrinos with another flare in 2014.
These concurrent spikes in gamma ray energy and neutrinos detected from a single source over the same period of time confirmed that blazars are the first known cosmic particle accelerators, researchers said.
Why it matters:
Neutrinos could be used to monitor nuclear proliferation, send messages through walls or even detect the ever-elusive dark matter. But in truth, physicists want to use them to discover more about the universe.
“Neutrinos provide us a way of exploring the dark side of the universe — regions from which electromagnetic radiation does not reach us,” said Olga Botner, an experimental particle physicist at Uppsala University and former IceCube lab spokesperson. “We can look at objects that we’ve never even seen before.”
Tracking neutrinos also helps physicists in their century-long pursuit to map the origins of cosmic rays — particles with the highest energies ever observed. Physicists think high-energy neutrinos started out as cosmic rays accelerated by the blazar. Knowing the locations of cosmic rays and neutrinos can helps scientists to understand the behaviors of interstellar magnetic fields and other elements our universe.
Editor’s note: The IceCube Neutrino Observatory is funded by the National Science Foundation, which is also a supporter of the PBS NewsHour.