Neutrinos are everywhere – trillions of the almost massless particles pass through your body every second – but they are notoriously difficult to attach, especially the rare high-energy ones from deep space. Only about a dozen of these cosmic neutrinos are discovered annually, and scientists had only linked one to its source. Now IceCube, the kilometer-wide neutron detector located deep below the South Pole, has traced another back to its distant 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,” said Tsvi Piran, a theorist at the Hebrew University of Jerusalem who was not involved in the study. The discovery suggests that these rare tidal disturbance events (TDEs) could be a major source of high-energy neutrinos and cosmic rays – other deep space visits whose origins have been a mystery.
The only way to spot neutrinos is to wait for something to hit. They do not often interact with matter, but very rarely do they collide head-on with an atomic nucleus, producing a soda of waste particles. when these particles subside, they emit a flash of light. To increase the chances of detecting these collisions, scientists need a huge amount of substance. IceCube fishes for them using a series of more than 5,000 photon detectors arranged in strings and sunk into 1 cubic kilometer of Antarctica. From the flash’s arrival time and brightness to each detector, scientists can calculate the direction a neutrino came from and whether its source is nearby or in deep space.
In 2017, IceCube discovered a long-lasting neutrino linked for the first time to an identifiable source: a superlight galaxy known as a blazar. Such galaxies contain lush, supermassive black holes in their centers; the matter they suck in burns so hot that it can be seen all over the universe. The process also creates a jet of high-velocity matter believed to be pointed directly at Earth.
On October 1, 2019, a flash in the detector revealed another likely deep candidate. As they do a few dozen times each year, IceCube scientists sent an alarm 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 that tears art, a nearby star, the team reports today in Natural astronomy. “When we saw that it could be a TDE, we immediately went ‘Wow!'”, Says lead author Robert Stein from 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 female – just like Earth’s tides on steroids. If it gets too close, gravity can tear the star up with half of its mass drawn into a hot light disk around the black hole, and the rest fly outward in a long streamer. It’s a process similar to what drives a blazar, but only lasts a few months. By cturing a neutrino from TDE, the team has now found evidence that TDEs can also feed a short-lived particle beam from the black hole, like a blazar burp.
This particular TDE was not new to astronomers. It had been discovered on April 9, 2019 by the Zwicky study and called AT2019dsg. The fact that this still supplied a neutrino-filled jet 150 days later was a surprise. “We could see that the source was really active, with a central engine running it for a long time,” Stein says.
Astrophysicists do not understand exactly how growing black holes drive these particle rays. But with two cosmic neutrinos now traced to them, jets emerge as a primary competitor to explain neutrinos in depth that precede neutron stars and stellar explosions. Jet is considered to produce neutrinos in the same way that particle physicists artificially produce neutrinos on Earth: with a high-energy beam of protons (the beam) that melts 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 to astrophysicists: the source of ultra-high-energy cosmic rays, particles like protons that zipper around the cosmos and bombard the Earth’s atmosphere daily. Making neutrinos requires acceleration of protons to high energy, Piran says, so that TDEs could produce the cosmic rays at the same time.
But Piran says there is some caution. Neutrino and TDE are only related to their position in the sky, and IceCube’s corrections are not very accurate. Stein admits that there is one in 500 chance that it is a random coincidence. Such odds will not impress particle physicists, who usually require the probability of one in several millions to require a discovery. “We will have to wait and see if there are more events,” says Stein. “I wish they had found two neutrinos,” says Piran, “then we would be in business.”