On Oct. 1, 2019, Earth was struck by an invisible, high-energy cosmic bullet moving at almost the speed of light. Trillions of these intergalactic bullets pass through our bodies every second without us even knowing, so there's no great concern for the planet. But this particular projectile was special. At the bottom of the world, the ghostly particle met its end after colliding with an ice molecule. Fortunately, it did so right next to an extremely sensitive detector embedded underneath the South Pole.
The detection set off an intergalactic hunt for the celestial gunslinger. What had fired the bullet?
In new research, published in the journal Nature Astronomy on Monday, scientists detail the detection of a subatomic particle -- known as a neutrino -- at the IceCube Neutrino Observatory in Antarctica. Using data from the Zwicky Transient Facility at California's Palomar Observatory, researchers were able to trace the origins of the subatomic bullet back to an extreme event some 700 million years ago: the cataclysmic destruction of a star as it was shredded by a .
It's the first time such an event has been linked to a neutrino detection.
Neutrinos are often described as "ghost particles" because the have no electric charge and have vanishingly small masses. Like light, they travel in basically a straight line from their destination. Other charged particles are at the mercy of magnetic fields, but neutrinos just barrel through the cosmos without impediment. We know they pour out of the core of the sun in huge quantities, and on Earth we can create them in nuclear reactors and particle accelerators.
In April 2019, the Zwicky facility detected a bright glow around a black hole some 700 million light-years away. The flare of light was produced when a star traveled too close to the black hole, which is around 30 million times more massive than the sun. The immense gravity of the black hole stretched the star and eventually it, ripped apart by the extreme forces. This is known as a "tidal disruption event," or TDE.
The violent end for the star is a brilliant beginning for astronomers. They were able to link the TDE to the detection of the neutrino by IceCube. The researchers theorize the TDE threw about half of the shattered star into space while the rest settled around the black hole in a gigantic "accretion disc" of hot, bright dust, gas and debris. The wild energies around the black hole in the disc result in huge jets of matter being shot out of the system. These jets can last for hundreds of days and could explain the small lapse in time between seeing the TDE and detecting the neutrino at IceCube.
Astrophysicists reason this shows the existence of a "central engine" that operates like a natural particle accelerator and can create high-energy neutrinos, some of which may collide with the Earth.
"The neutrino emerged relatively late, half a year after the star feast had started," said Walter Winter, a theoretical astrophysicist with the German Electron Synchrotron, or DESY. "Our model explains this timing naturally."
Winter and co-author Cecilia Lunardini published their modeling in the same issue of Nature Astronomy on Monday.
The discovery of a neutrino emanating from a TDE is a breakthrough for astronomers hoping to understand the universe in new ways. Scientists have only been able toonce previously. It was IceCube that made that detection, too. In 2017, researchers at the observatory detected the telltale signature of a neutrino and alerted astronomers to the phenomenon. Telescopes were able to trace the source of the neutrino back to a distant galaxy that housed a "blazar" -- a huge black hole surrounded by an accretion disc with a jet aimed directly at the observer.
The two detections both show that black holes are intergalactic gunslingers, firing ghost particles from deep space across the universe. This could help give astronomers insight on the processes occurring close to a black hole and could even begin to solve a mystery that's haunted astrophysics since the 1960s: where do the ultra-high-energy cosmic rays that sometimes smash into Earth's atmosphere come from?
Researchers have detected a number of TDE's since the Zwicky Transient Facility began surveying the skies, and in the future more sensitive telescopes may be able to further link these high-energy particles to the events. IceCube will also be critical for improving our understanding. The observatory is set to get an upgrade during the 2022 and 2023 Antarctica seasons, pandemic notwithstanding, which should increase the number of neutrino detections by a factor of 10.
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