A ghostly particle that crashed into Antarctica in 2019 can be traced back to a black hole ripping apart a star while acting as a giant cosmic particle accelerator, a new study finds.
The scientists investigated a kind of subatomic particle known as a neutrino, which is generated by nuclear reactions and the radioactive decay of unstable atoms. Neutrinos are extraordinarily light, about 500,000 times lighter than the electron.
Neutrinos have no electrical charge and rarely interact with other particles. As such, they can slide through matter easily: a light year of lead, equivalent to about 5.8 trillion miles (9.5 trillion kilometers), would only stop about half of the neutrinos that pass through it.
However, neutrinos occasionally hit atoms. When that happens, they emit telltale flashes of light, which scientists have previously detected to confirm their existence.
In the new study, the researchers examined an extremely high-energy neutrino they saw on October 1, 2019, using the IceCube Neutrino Observatory at the South Pole.
“It crashed into the Antarctic ice with a remarkable energy of more than 100 tera-electron volts,” study co-author Anna Franckowiak, now at the University of Bochum in Germany, said in a statement. “For comparison, that’s at least 10 times the maximum particle energy that can be achieved in the world’s most powerful particle accelerator, the Large Hadron Collider.”
Video: Neutrino goes back to the black hole that destroyed a star
Related: The strange behavior of neutrinos could explain the ancient mystery of antimatter
To discover the origins of such a powerful neutrino, scientists mapped its way through space. They found that it likely came from the galaxy designated “2MASX J20570298 + 1412165” in the constellation Delphinus, the dolphin, and is located about 750 million light-years from Earth.
About six months before scientists detected the high-energy neutrino, astronomers witnessed a glow from this galaxy using the Zwicky Transitional Facility on Mount Palomar in California. This light is likely coming from a black hole ripping apart a star, known as a tidal disruption event called “AT2019dsg.”
The researchers suggest that a star got too close to a supermassive black hole in the center of the galaxy 2MASX J20570298 + 1412165, one about 30 million times more massive than the sun. It was then ripped apart by the colossal gravity of the black hole, an extreme version of the way the moon makes the tides rise and fall on Earth.
The scientists noted that about half of the star’s debris was thrown into space, while the other half settled on a spinning disk around the black hole. As matter from this dismantled star fell into this disk, it became hotter and brighter enough for astronomers to see it from Earth.
The researchers estimated that this neutrino only had a 1 in 500 chance of coinciding with the event. This suggested that scientists likely detected the first particle dating back to a tidal disruption event.
“For a long time it was predicted by theoretical work that neutrinos could come from tidal disruption events,” study lead author Robert Stein, a multi-messenger astronomer at the German Electron Synchrotron (DESY), told Space.com. Zeuthen, Germany. “This work is the first observational evidence to support that claim.” He and his colleagues detailed their findings online February 21 in the journal Nature Astronomy.
These new findings shed light on tidal disruption events, about which much is unknown. Specifically, the researchers suggested that the neutrino came from jets of matter that shot out near the black hole’s accretion disk at nearly the speed of light, Cecilia Lunardini, a particle astrophysicist at Arizona State University, told Space.com. She and study co-author Walter Winter in DESY detailed their findings online February 22 in a companion study in the journal Nature Astronomy.
Although these relativistic jets likely spewed out many different types of particles, they were mostly electrically charged particles, which are deflected by intergalactic magnetic fields before they can reach Earth. In contrast, neutrinos (which have no charge) can travel in a straight line like light rays from the tidal disruption event.
This discovery marks only the second time that scientists have traced a high-energy neutrino back to its origin, Stein said. The first time, in 2018, astronomers traced such a neutrino to blazar TXS 0506 + 056, a huge elliptical galaxy with a fast-spinning supermassive black hole at its heart.
“Knowing where high-energy neutrinos come from is a big question in particle astrophysics,” Stein said. “We now have more evidence that they could probably come from tidal disruption events.”
A strange aspect of this discovery was how the neutrino was not detected until half a year after the black hole began to devour the star. What this suggests is that the tidal disruption event can act as a giant cosmic particle accelerator for months, Stein said.
Although the researchers only detected one neutrino from this tidal disruption event, “for us to detect even one, there must have been billions and billions that it was generating,” Stein said. “We were lucky to see one.”
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