Using a neutrino detector made of Antarctic ice, astronomers have for the first time pinpointed the source of a handful of high-energy neutrinos from far beyond our galaxy: a powerful blazar shining like a beacon from nearly 4 billion light-years away.
The extragalactic neutrinos and their origins, described in two papers in the journal Science, shed light on the century-old question of where cosmic rays come from and offer the first clear proof of the potential for this nascent brand of astronomy.
"We are not going to solve high-energy astrophysics in the old-fashioned way anymore," said Francis Halzen, a particle astrophysicist at the University of Wisconsin-Madison and principal investigator for IceCube, the frozen observatory that made the discovery.
Neutrinos are exceedingly tiny particles, weighing at less than one ten-billionth the mass of a proton. Many billions of these subatomic particles pass through your fingertip every second. Even though they're plentiful, neutrinos don't interact much with matter, passing through planets, stars, even entire galaxies like speeding subatomic phantoms.
But astronomers hunt for neutrinos anyway, partly because they've suspected that they could solve the mystery of the origins of the cosmic rays that bombard Earth from space.
Highly energetic charged particles
Cosmic rays are highly energetic charged particles, mostly protons, that have been revved up to enormous energies and hurled across the universe. It would take a powerful cosmic engine – say, a supermassive black hole at a galaxy's heart, or an enormous supernova – to accelerate these atomic fragments to such high energies.
But until now, scientists haven't known for sure where these cosmic rays come from. That's because as they travel intergalactic distances, their paths are warped by the magnetic fields that permeate space – which means that by the time they get to Earth, they're no longer pointing back at their source.
Neutrinos offer a solution to this problem because these neutral particles are unaffected by magnetic fields. By the time they reach Earth, they're still pointing the way home. On top of that, the kinds of powerful cosmic forges that would generate high-energy cosmic rays also would produce a torrent of high-energy neutrinos.
But the very quality that makes these ghostly particles so useful – the fact that they don't interact with matter – also makes neutrinos exceedingly difficult for scientists to catch in action. For every individual high-energy neutrino hit, Halzen said, roughly 10,000 or 100,000 more pass through unscathed.
The IceCube collaboration set out to detect that rare, singular neutrino strike. Composed of more than 5,000 sensors embedded in a cubic kilometer of ice sitting deep beneath the Antarctic surface, IceCube picks up the flashes of blue light caused by secondary particles after a neutrino makes contact. The scientists can analyze that resulting light track to tell what direction the particle came from and how energetic it was when it hit.
In 2013, the collaboration announced it had found 28 high-energy neutrinos that had originated from deep space, but the group was not able to tell where exactly any of them came from.
Then, on Sept. 22, 2017, the scientists picked up an energetic neutrino that had clearly originated far outside our interstellar neighborhood. Gamma ray and X-ray telescopes turned toward the source, picking up a light signal across the electromagnetic spectrum. The light was coming from a blazar named TXS 0506+056, a giant elliptical galaxy with a black hole at the center that's gobbling up material and shooting out twin beams of light on either side of its disk, one of which is pointed directly at Earth like the beam of a flashlight.
Still, there was a small chance – about 1 in 1,000 – that the neutrino's apparent origin and the blazar signal were mere coincidence. So the researchers went back in the archives, looking for previous neutrino measurements that also could have come from the blazar's direction.
Sure enough, the researchers found more than a dozen neutrinos from September 2014 to March 2015 that appeared to be coming from the direction of the blazar. Those results were published in a second paper in Science.
"In my opinion, this is as significant as the first steps in X-ray astronomy, which were awarded the Nobel Prize," said Alexander Kusenko, a particle astrophysicist at UCLA who was not involved in the study.
Such neutrino discoveries could help astronomers to better understand the inner workings of these cosmic events, Kusenko said.
It also may allow scientists to see old events in a new light, Halzen said.
For one thing, it would have taken an extremely powerful source to push these particles to such high energies and then send them across nearly 4 billion light years, he pointed out.
"So there's something special about this source," Halzen said – something special that was not obvious from the blazar's light profile and which will require further study to understand.
Already, he added, neutrino astronomy is revealing extraordinary events right in front of scientists' eyes.