Rare cosmic neutrino traced to star-swallowing black hole

When a supermassive black hole tears a star apart (imagined here), it produces copious light—and maybe neutrinos, too.

DESY/Science Communication Lab

Neutrinos are everywhere—trillions of the virtually massless particles pass through your body every second—but they’re notoriously hard to pin down, especially the rare high-energy ones from deep space. Only about a dozen of these cosmic neutrinos are detected annually, and scientists had connected only one to its source. Now, IceCube, the kilometer-wide neutrino detector nestled deep beneath the South Pole, has traced another one back to its far-flung birthplace: a supermassive black hole tearing a star to pieces in a galaxy 750 million light-years away.

“It’s a very exciting story if this is correct,” says Tsvi Piran, a theorist at the Hebrew University of Jerusalem who was not involved in the study. The discovery suggests these rare tidal disruption events (TDEs) could be a major source of high-energy neutrinos and cosmic rays—other deep-space visitors whose origins have been a mystery.

The only way to detect neutrinos is to wait for one to hit something. They don’t often interact with matter, but very rarely they will collide head on with an atomic nucleus, producing a shower of debris particles; as these particles decelerate, they emit a flash of light. To boost the chances of detecting these collisions, researchers need a huge volume of matter. IceCube fishes for them using an array of more than 5000 photon detectors arranged in strings and sunk into 1 cubic kilometer of Antarctic ice. From the arrival time and brightness of the flash at each detector, researchers can calculate the direction a neutrino came from and whether its source is nearby or in deep space.

In 2017, IceCube detected a long-traveled neutrino that, for the first time, was linked to an identifiable source: a superbright galaxy known as a blazar. Such galaxies contain voracious supermassive black holes in their centers; the matter they suck in burns so hot that it can be seen across the universe. The process also creates a jet of high-velocity matter thought to be pointed straight at Earth.

On 1 October 2019, a flash in the detector revealed another likely deep-space candidate. As they do a few dozen times each year, IceCube researchers sent out an alert so astronomers could scan the sky in the direction of the arriving neutrino. A California telescope, the Zwicky Transient Facility, swung into action and found that it was a TDE, a supermassive black hole tearing apart a nearby star, the team reports today in Nature Astronomy. “When we saw it could be a TDE, we immediately went ‘Wow!’” says lead author Robert Stein of the DESY particle physics laboratory in Germany.

TDEs remain something of a mystery; fewer than 100 have been seen so far. When a star orbits close to a supermassive black hole, the intense gravity distorts its shape—like Earth’s tides on steroids. If it gets too close, the gravity can rip the star up, with half its mass pulled into a hot bright disk around the black hole and the rest flying outward in a long streamer. It’s a similar process to what powers a blazar, but lasts just a few months. By capturing a neutrino from the TDE, the team has now found evidence that TDEs can also feed a short-lived particle jet from the black hole, like a blazar burp. 

This particular TDE was not new to astronomers. It had been discovered on 9 April 2019 by the Zwicky survey, and dubbed AT2019dsg. The fact that this one was still powering a neutrino-filled jet 150 days later was a surprise. “We could see the source was really active, with a central engine powering it for a long time,” Stein says.

Astrophysicists don’t understand exactly how accreting black holes power these particle jets. But with two cosmic neutrinos now traced to them, jets are emerging as a primary contender for explaining deep-space neutrinos, edging ahead of neutron stars and stellar explosions. Jets are thought to produce neutrinos in much the same way that particle physicists artificially make neutrinos on Earth: with a high-energy beam of protons (the jet) that slams into surrounding material, explains co-author Suvi Gezari of the Space Telescope Science Institute, who first discovered AT2019dsg. “For TDEs to emerge as a likely site for neutrino production is very exciting,” she says.

This could be an important clue in another mystery for astrophysicists: the source of ultra–high-energy cosmic rays, particles like protons that zip around the cosmos and bombard Earth’s atmosphere daily. Making neutrinos requires accelerating protons to high energy, Piran says, so TDEs could be producing the cosmic rays at the same time.

But Piran says some caution is due. The neutrino and the TDE are linked only by their position in the sky, and IceCube’s fixes are not very precise. Stein concedes there is a one in 500 chance it’s a random coincidence. Such odds won’t impress particle physicists, who usually require a likelihood of one in several million to claim a discovery. “We will have to wait and see if there are additional events,” Stein says. “I wish they had found two neutrinos,” Piran says, “then we would be in business.” 

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