A black hole shredded a star in a distant galaxy and shot high-energy subatomic particles into space. We found a particle on Earth.

In 2019, an invisible, high-energy cosmic bullet moving at almost the speed of light struck the Earth. Scientists call this subatomic particle a neutrino.

Trillions of these neutrinos pass through our bodies every second without us even knowing. So, don’t worry. The planet is just fine.

The researchers traced this ghostly particle to a doomed star that came too close to the central supermassive black hole of a distant galaxy. The black hole’s enormous gravity ripped it apart.

“The force of gravity gets stronger and stronger, the closer you get to something. That means the black hole’s gravity pulls the star’s near side more strongly than the star’s far side, leading to a stretching effect,” said Robert Stein, a scientist at the Deutsches Elektronen-Synchrotron particle accelerator research center.

“This difference is called a tidal force, and as the star gets closer, this stretching becomes more extreme. Eventually, it rips the star apart, and then we call it a tidal disruption event. It’s the same process that leads to ocean tides on Earth, but luckily for us the moon doesn’t pull hard enough to shred the Earth.”

The work, which included researchers from more than two dozen institutions, including New York University and Germany’s DESY research center, focused on neutrinos — subatomic particles that are produced on Earth only in powerful accelerators.

Neutrinos — as well as the process of their creation — are really hard to detect, making their discovery noteworthy.

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This is the first particle that scientists can trace back to such a ‘tidal disruption event’ (TDE). It provides evidence that these little-understood cosmic catastrophes can be powerful natural particle accelerators, Robert Stein reports in the journal Nature Astronomy. The observations also demonstrate the power of exploring the cosmos via a combination of different ‘messengers’ such as photons (the particles of light) and neutrinos, also known as multi-messenger astronomy.

This high-energy neutrino traveled over a period of 700 million years to reach us. That’s the time when the first animals developed.

The gargantuan black hole that shredded the star lies at the heart of an unnamed galaxy (cataloged as 2MASX J20570298+1412165) in the constellation Delphinus (The Dolphin). The black hole is as massive as 30 million suns. And its gravity is so intense that it tears stars to pieces.

Credit: DESY, Science Communication Lab

The researchers suggest the tidal disruption event shot about half of the shattered star into space. Meanwhile, the rest settled around the black hole in an enormous “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. Before plunging into oblivion, the matter from the accretion disc gets hotter and hotter and shines brightly. Scientists spotted the glow from Earth in April 2019, at the Zwicky Transient Facility in California.

Half a year later, on 1 October 2019, the IceCube neutrino detector at the South Pole registered an extremely energetic neutrino from the direction of the tidal disruption event. It smashed into the antarctic ice with at least 10 times the energy of any particle accelerator on Earth.

The analysis showed this particular neutrino had only a one in 500 chance of being purely coincidental with the TDE. The detection prompted further observations.

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Scientists captured the event through a variety of different telescopes – radio, optical and ultraviolet. Thus, they were able to gather much more detail about it than they would otherwise. This “multi-messenger” astronomy allows for a much greater understanding of events happening in space.

Astrophysicists reason this discovery 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 cosmic accelerator throws out different types of particles. But apart from neutrinos and photons, these particles are electrically charged. And thus deflected by intergalactic magnetic fields on their journey. Only the electrically neutral neutrinos can travel on a straight line like light from the source towards Earth and so become valuable messengers from such systems.

“The combined observations demonstrate the power of multi-messenger astronomy,” says co-author Marek Kowalski, head of neutrino astronomy at DESY and a professor at Humboldt University in Berlin. “Without the detection of the tidal disruption event, the neutrino would be just one of many. And without the neutrino, the observation of the tidal disruption event would be just one of many. Only through the combination could we find the accelerator and learn something new about the processes inside.”

Scientists found the association of the high-energy neutrino and the tidal disruption event by using a sophisticated software package called AMPEL, specifically developed at DESY to search for correlations between IceCube neutrinos and astrophysical objects detected by the Zwicky Transient Facility.

Researchers have detected several TDE’s since the Zwicky Transient Facility began surveying the skies. 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. This should increase the number of neutrino detections by a factor of 10.

Scientists reported their discovery in the journal Nature Astronomy.

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