Researchers have been able to trace the path of a neutrino from a distant galaxy – over nearly four billion light-years away.
Outside the astronomical world, no one has noticed what happened on September 22, 2017 at the South Pole. The event was also extremely difficult to grasp: a neutrino – one of those enigmatic elementary particles that haunt the earth as part of cosmic radiation – has been tracked down with the help of an elaborate experiment.
But not only that, with a timely observational campaign of more than a dozen earth and space observatories, the path of that one neutrino could be traced back to its source. As reported by an international research team, the source of this particle lies in a galaxy nearly four billion light years away: it is a gigantic black hole in the constellation of Orion.
Never before has such an experiment been successful. The new results may not only be a key to understanding these exotic particles, they are also an important step towards a new, more comprehensive kind of astronomy . “The observation of cosmic neutrinos allows insights into processes that were previously invisible,” says Klaus Helbing of the University of Wuppertal in Germany, who was involved in the work.
Neutrinos are among the most common elementary particles, but at the same time are extremely puzzling. They are constantly bombarding our planet in unimaginable numbers. These low-mass, electrically neutral particles penetrate matter almost unhindered. This characteristics make neutrinos a blessing for science: since they can not be stopped by anything, they whiz through stars, planets and entire galaxies and reach us as witnesses of astronomical events from which no electromagnetic waves can reach us.
However, they are extremely difficult to measure for this very same reason. At the South Pole, researchers at the Icecube South Pole Neutrino Observatory have been exploiting the effect that neutrinos have on atoms in the ice for several years. When this happens, charged particles are created that produce light — and that can be measured.
This is what happened on September 22nd last year. The neutrino we’re talking about had an energy of about 300 teraelconds — more than 40 times what the protons in the largest particle accelerator in the world have reached. This enormous energy showed that the particle must come from a distant celestial object: According to current knowledge, energy-rich neutrinos are formed as by-products in “cosmic particle accelerators” such as matter jets from black holes or exploding stars.
Hoping to clarify the origin of the neutrino, numerous observatories were immediately alerted to look in the direction of the particle’s momentum. The eligible sky region was investigated across the electromagnetic spectrum — with success: “We have seen an active galaxy, a large galaxy with a huge black hole in the center,” said Marek Kowalski, head of neutrino astronomy at the Deutsches Elektronen-Synchrotron in Germany. “This is a milestone, we are opening a new window into the high-energy universe.”
The observed active galaxy nucleus, a so-called blazar with the catalog number TXS 0506 + 056, is located in the constellation Orion, scarcely four billion light years from the earth. The neutrino source may have been a jet that shot out into space as the black hole devoured matter. This eruption of the blazar could also be detected with other observations – from radio radiation to gamma radiation.
Of course, the neutrino from there was not alone on the way to the South Pole: In a second study, the researchers identified in their data archives numerous other neutrinos, which had apparently come in recent years also from the direction of TXS 0506 + 056. The researchers hope to be able to look even better into the universe’s past through the lens of such events.