‘Ghost particles’ introduce new era for astrophysics
Astronomers from around the world announced a major discovery on Thursday that could help scientists better understand the birth of the universe and some of the most exotic objects in it, including black holes.
By linking a massive detector buried under the South Pole, a command center at Pennsylvania State University, advanced satellites, and several land-based observatories, a team of hundreds was able to pinpoint the first known cosmic source of a special kind of neutrino, a particle that passes through virtually all matter on Earth.
Using neutrinos marks the beginning of a new era of astrophysical research that doesn’t rely solely on light, said France Cordova, director of the National Science Foundation, in a statement. The findings were published as two articles in Science on Thursday.
For centuries, people have been “looking at the night sky, and they saw photons – which is light – hitting their eyes,” explained Drexel University assistant professor Naoko Kurahashi Neilson, who helped lead efforts to locate the source of the high-energy neutrinos.
Modern astronomy, too, “is all about looking at the universe with light,” she said. But light can be blocked, either by objects in space or by clouds here on Earth. And many of the most exotic cosmic objects don’t emit photons, making them exceedingly difficult to study.
“Our dream,” Kurahashi Neilson said, “was to do (astronomy) with neutrinos.”
It turns out the first known source of high-energy neutrinos is one of the most recognizable constellations in the sky: Orion, the archer.
Just off Orion’s left shoulder, some 3.8 billion light-years away, floats a galaxy that is being sucked into a supermassive black hole. As that galaxy’s gas, dust, stars, and possible planets grind together, jets of x-rays, radio waves, and ultra-high energy particles blast out. This hot astronomical mess is known as a blazar.
On Sept. 22, 2017, a single high-energy neutrino from that blazar banged into detectors buried more than a kilometer underground at the South Pole. That kickstarted a flurry of activity in more than a dozen countries. The signal was wired to the University of Wisconsin, then routed to a command center at Penn State which alerted more than a dozen other observatories and satellites around the world, each hoping to catch further signals that would help pin down the source.
“I actually was on maternity leave,” Kurahashi Neilson said, “but of course you just have to go back when that happens.” She and her team at Drexel began poring over nearly a decade’s worth of round-the-clock data from IceCube, the detector at the South Pole. Orion’s left shoulder had indeed been active before.
“I was on a plane, about to take off (to Europe),” said Azadeh Keivani, an astrophysicist who helped coordinate satellites for the discovery while at Penn State. She is now at Columbia University. Keivani confirmed the signal on the tarmac and sent it to NASA’s Swift satellite overhead.
Launched in 2004 and operated by a crew out of Penn State, Swift was the first responder to home in on the source.
NASA’s Fermi telescope soon followed, prompting a prolonged “observation campaign,” said Keivani. Detection of X-rays, gamma rays, and UV light all helped confirm that the blazar was active, and gave the researchers confidence that they were looking at a true source.
Drexel, Penn State, the University of Delaware, Massachusetts Institute of Technology, and NASA were among the dozens of contributing institutions.
These and other high energy neutrinos will help “complete the picture of the universe,” said Keivani. “But we have to collaborate more. We are just at the beginning.”
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