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Astronomers trace the source of a high-energy particle that slammed into Earth

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Finding the birthplace of a deep-space neutrino

A rendering of the IceCube Observatory in Antarctica detecting neutrinos.
Image: IceCube / NSF

Astronomers may have discovered the deep-space origin of a mysterious high-energy particle that plunged straight through Earth last year. The tiny particle, known as a neutrino, seems to have come from a hyperactive black hole located 4 billion light-years away. It’s the first time researchers have pinpointed the possible origin for one of these high-energy neutrinos, bringing scientists closer to figuring out the objects that produce these strange lightweight particles that fill the Universe.

In September, researchers working near the South Pole detected the presence of a super high-energy neutrino in the Antarctic ice. These fast-moving particles often zip right through objects like our planet without ever leaving a trace that they were there. But this visiting neutrino was a rare breed: it actually bumped into the ice, leaving a trail that the researchers were able to measure with their observatory, IceCube. The team then quickly mobilized to home in on the patch of sky that the particle came from.

In that patch, they found a possible culprit for the neutrino: an overactive galaxy with a supermassive black hole at its center. This kind of galaxy is actually known as a blazar, which means its black hole core is spewing radiation (and other stuff) in the direction of Earth. The discovery, detailed today in two papers in Science, serves as strong evidence that the neutrino originated from this black hole. That’s huge since astronomers have never been able to pinpoint the potential birthplace of a such a high-energy neutrino before. But now, blazars could be good places to look for neutrinos like this one in the future.

If we know where neutrinos come from, scientists might be able to use them as tools for probing the cosmos. Neutrinos are thought to arise inside some of the most extreme objects in the Universe, such as dying stars, black holes, and colliding galaxies. By confirming the creators of neutrinos, astronomers could then use these particles the same way we use X-rays to look inside our own bodies. “By looking for neutrinos, we can learn more about what’s going on inside these objects,” Dawn Williams, an associate professor of physics and astronomy at the University of Alabama and one of the members of IceCube team who made the discovery, tells The Verge. “That can add to our knowledge of these objects, which are still very much a subject of study.”

A rendering of the blazar sending gamma rays and neutrinos to Earth.
Image: IceCube / NASA

Harnessing the power of neutrinos is tough, though, as they are considered some of the stealthiest particles in the Universe. They’re the lightest fundamental particle that we know about, with a mass just above zero. But unlike other particles, such as electrons or protons, neutrinos don’t have a charge, so they’re not affected by things like magnetic fields. In fact, they’re barely affected by anything at all. Neutrinos can travel in a straight line through the Universe, covering vast distances, without deviating from their course. They’re so small that they just pass right through planets, stars, and galaxies like diminutive ghosts. They’re passing through you right now; it’s estimated that trillions of neutrinos pass through a person’s body each second.

But what neutrinos lack in size, they make up for in energy. Astronomers believe that neutrinos are created during violently energetic processes like nuclear fusion reactions, which send these particles streaming outward at close to the speed of light. So everything from exploding stars to nuclear bombs can create these elusive little objects. It’s also believed that most of the neutrinos in the Universe were created just after the Big Bang and now permeate the cosmos.

Before today, scientists knew of three different sources for neutrinos that regularly hit Earth. We’ve picked up these particles coming from inside our Sun, and we can also sometimes measure ones that are coming from our own atmosphere. Other kinds of energetic particles from outside our galaxy, known as cosmic rays, pelt our atmosphere, breaking up molecules into pieces and producing showers of neutrinos over the Earth. And just once in 1987, astronomers detected an excess of neutrinos coming from a supernova just outside our galaxy.

Since neutrinos are so stealthy, it takes a very special kind of detector to find these particles. One of the best facilities is the IceCube Neutrino Observatory near the South Pole. It’s made up of thousands of light-sensitive tubes embedded in the ice sheet that are capable of measuring the very rare neutrinos that actually collide with the Earth. “They have a very small probability of interacting,” Erik Blaufuss, a professor of physics at the University of Maryland and member of the IceCube discovery team, tells The Verge. “That’s why we have to build such a large instrument at the South Pole.” Every so often, a neutrino won’t pass right through our planet but will chip a part of an atom in the Antarctic ice. When that happens, it basically destroys the atom’s nucleus, creating a shower of blue light that travels across the transparent ice. That light shower is what the detector picks up. Depending on the trail, IceCube can figure out a neutrino’s energy and the direction it was traveling.

The IceCube observatory expected to see neutrinos from the atmosphere. But in 2013, the astronomers noticed that they were picking up particles that were millions of times more energetic than the ones produced by the Sun or even the ones found from the 1987 supernova. These high-energy neutrinos were rarer than other kinds, too: IceCube estimates that they pick up about 10 of this type each year. The researchers strongly suspected that these neutrinos are coming from far outside our Solar System and galaxy, but they didn’t have proof.

The neutrino that hit in September was one of these high-energy types. And when IceCube detected it, the team immediately sent out an alert to other telescopes to see if they might be able to find the source of the particle. IceCube told other astronomers which part of the sky the neutrino came from so that they knew where to point their telescopes. Around 20 observatories obliged. Two of them, NASA’s Fermi space telescope and the MAGIC telescope in the Canary Islands, measured a large surge of high-energy gamma rays coming from the blazar in that part of the sky. The findings indicated that the blazar was sending out highly energetic material at the time of the detection, and it just may have sent out the neutrino as well.

A rendering of the blazar sending a jet of radiation outward.
Image by Nate Follmer / Penn State University

Then the IceCube team decided to look back through their archives to see if they had any more evidence to back this up. They found that between 2014 and 2015, the detector had picked up a bunch of neutrinos coming from this same area of the sky. All of this doesn’t decisively prove that the blazar is to blame, but it’s still the best explanation so far. We would need to do more observations to get a more statistically significant discovery,” says Williams. “But it’s all very exciting because these are independent checks, and we’d never seen this close of an association between gamma rays and neutrinos before.”

The IceCube team hopes to do more collaborations like this where they alert other light telescopes to point in the direction that an interesting neutrino came from. Known as multimessenger astronomy, it’s a way of doubling up and using two different kinds of signals — light and neutrinos — to confirm sources in the sky. “By providing that ability to focus in on a particular patch of sky at a particular time, we increase the sensitivity of the facility, and we increase the chances of a detection,” Derek Fox, an associate professor of astronomy at Penn State University, who was not part of the IceCube team that made the discovery but contributed to one of the Science papers, tells The Verge.

That could increase our chances of finding neutrino sources in the future. And maybe one day, astronomers could observe distant objects a new way: by studying the strange fundamental particles they send to Earth. “Humans have observed the Universe using light for literally our entire history as a species,” says Fox. “So now we’re just reaching a point here in 2017 and 2018 where we routinely expect to detect cosmic sources by means other than light.”