Cosmic neutrino blast

A little bit more is now known about the cosmos, thanks to research involving a neutrino space odyssey, a black hole devouring a sun, and an IceCube.

Researchers from Deutsches Elektronen-Synchrotron (DESY), Germany, detected the ghost of a star in the form of the elusive neutrino particle, flung from the debris of a black hole that devoured a star.

The neutrino is the first to be discovered from this kind of event, which suggests the celestial collision also acted as a gargantuan cosmic particle accelerator.

In their paper, published in Nature Astronomy, the researchers estimated that the carnivorous black hole was about as big as 30 million suns, and the immense gravitational pull caught the sun and pulled it in. But the force across the star was not even, leading to a distorted shape and eventual disintegration.

“The force of gravity gets stronger and stronger, the closer you get to something,” explains Robert Stein of DESY and Humbolt University, who led the study. “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.

“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.”

About half of the obliterated star debris settled as a swirling disc around the black hole due to its massive gravity, while the rest was flung off into space.

Back inside of the swirling disc – called the accretion disc – the star-dust particles were smashing together and making new particles. This “engine” shot off jet-like bursts of matter and particles, one of which was the travelling neutrino.

Most of the spewed particles from these events are electrically charged, and get pulled and pushed by other magnetic fields in space and have trouble making it to Earth. The neutrally charged neutrino, however, was able to travel directly, even right through the heart of other galaxies.

The neutrino’s odyssey began in such a tidal disruption event least 700 million years ago, in an unknown galaxy within the Delphinus constellation. In October 2019, it was registered by the IceCube neutrino detector at the South Pole.

“It smashed into the Antarctic ice with a remarkable energy of more than 100 teraelectronvolts,” says co-author Anna Franckowiak, from DESY, who is now a professor at the University of Bochum.

“For comparison, that’s at least ten times the maximum particle energy that can be achieved in the world’s most powerful particle accelerator, the Large Hadron Collider at the European particle physics lab CERN, near Geneva.”

The IceCube neutrino detector picks up neutrinos regularly, but it isn’t always clear where they come from.

In this case, though, the previous detection of the tidal disruption event helped the team realise that the two events were linked. They estimated there was only a 1 in 500 chance that the neutrino’s journey and the cosmic devouring were coincidentally linked.

“This is the first neutrino linked to a tidal disruption event, and it brings us valuable evidence,” explains Stein.

“Tidal disruption events are not well understood. The detection of the neutrino points to the existence of a central, powerful engine near the accretion disc, spewing out fast particles.”

Their new model sheds light on the age-old question of where ultra-high energy cosmic rays originate.

As the star approaches the black hole, the enormous tidal forces stretch it more and more until it is finally shredded. Half of the stellar debris is flung back into space, while the remaining part forms a rotating accretion disk from which two strong outflows of matter shoot up and down. The system acts as a powerful natural particle accelerator. Credit: DESY, Science Communication Lab.