When a supermassive black hole tears a star apart (pictured here), it produces abundant light – and perhaps neutrinos as well.
DESY / Science Communication Laboratory
By Daniel Clery
Neutrinos are everywhere – trillions of the virtually massless particles pass through your body every second – but they are notoriously difficult to pin down, especially the scarce high-energy units from deep space. Only about a dozen of these cosmic neutrinos are detected annually, and scientists have linked only one to the source. IceCube, the kilometer-wide neutrino detector located deep beneath the South Pole, traced another 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 it’s correct,” says 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) may be a major source of high-energy neutrinos and cosmic rays – other visitors in deep space whose origins were a mystery.
The only way to detect neutrinos is to wait until someone hits something. They do not often interact with matter, but they rarely collide with an atomic nucleus and cause a shower to dump particles; if these particles slow down, they emit a flash of light. To increase the chances of detecting these collisions, researchers need a large amount of matter. IceCube fishes for them using a range of over 5000 photon detectors arranged in strings and sunk into 1 cubic kilometer of Antarctic ice. From the arrival time and the brightness of the flash at each detector, researchers can calculate the direction from which a neutrino came and whether the source is near or in deep space.
In 2017, IceCube detected a long-range neutrino that was first linked to an identifiable source: a super-bright galaxy known as a jacket. Such galaxies contain greedy 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 matter at a high velocity that is pointed straight at the earth.
On October 1, 2019, a flash in the detector reveals another likely candidate for the deep space. As they do several dozen times each year, IceCube scientists sent out a warning so that astronomers could look at the sky in the direction of the neutrino. A telescope in California, the Zwicky Transient Facility, swung into action and found that it was a TDE, a supermassive black hole tearing a star in the area, the team reported today in Natural Astronomy. “When we saw that it could be a TDE, we immediately said ‘Wow!’ Gone, ”says lead author Robert Stein of the DESY Particle Physics Laboratory in Germany.
TDEs remain something of a mystery; less than 100 have been seen so far. When a star orbits near a supermassive black hole, the intense gravity distorts its shape – like the tides of the earth on steroids. If it gets too close, gravity can advance the star, with half of its mass drawn in a hot clear disk around the black hole and the rest fly outward in a long spreader. It’s a similar process to driving a jacket, but it only takes a couple of months. By capturing a neutrino from the TDE, the team has now found evidence that TDEs can also carry a transient particle beam out of the black hole, such as a blowpipe.
This particular TDE was not new to astronomers. It was discovered on April 9, 2019 by the Zwicky survey and named the name AT2019dsg. The fact that this 150 days later still propelled a neutrino-filled jet was a surprise. “We could see that the source was really active, with a central engine that powered it for a long time,” Stein says.
Astrophysicists do not understand exactly how the growth of black holes drives these particle rays. But with two cosmic neutrinos now being traced hereafter, radiators come as a primary contender for the declaration of neutrinos in space, which precedes the neutron stars and stellar explosions. Rays are thought to produce neutrinos in the same way that particle physicians artificially produce neutrinos on Earth: with a high-energy beam of protons (the beam) striking the surrounding material, explains co-author Suvi Gezari of the Space Telescope Science Institute. , who first discovered AT2019dsg. “It’s very exciting to reveal TDEs as a likely site for neutrino production,” she says.
This could be an important clue in another mystery to astrophysicists: the source of ultra-high-energy cosmic rays, particles like protons zipping around the cosmos, and the earth’s atmosphere bombing daily. To make neutrinos, protons must accelerate to high energy, says Piran, so TDEs can produce the cosmic rays at the same time.
But Piran says some caution is needed. The neutrino and the TDE are only linked by their position in the air, and IceCube’s solutions are not very precise. Stein admits that the chance at one in 500 is that it is coincidental. Such a chance would not impress particle physicists, who usually need the probability of one in a few million to claim a discovery. “We’ll have to wait and see if there are additional opportunities,” Stein says. “I wish they found two neutrinos,” Piran says, “then we’ll be busy.”