A single subatomic particle detected at the South Pole last September is helping to solve a major cosmic mystery: what creates electrically charged cosmic rays, the most energetic particles in nature.

Follow-up studies by more than a dozen observatories suggest that researchers have, for the first time, identified a distant galaxy as a source of high-energy neutrinos

This discovery could, in turn, help scientists pin down the still mysterious source of protons and atomic nuclei that arrive to Earth from outer space, collectively called cosmic rays. The same mechanisms that produce cosmic rays should also make high-energy neutrinos.

Multiple teams of researchers from around the world describe the neutrino's source in at least seven papers released on 12 July.

“Everything points to this as the ultra-bright, energetic source — a gorgeous source.” says Elisa Resconi, an astroparticle physicist at the Technical University of Munich in Germany.

Astrophysicists have proposed a number of scenarios for astrophysical phenomena that could produce both high-energy neutrinos and their electrically charged counterparts: protons and atomic nuclei collectively called cosmic rays. But until now, they had not managed to unambiguously trace any of these particles back to their source. This is especially difficult with cosmic rays, whose electric charges make their paths curve on their way to Earth, whereas neutrinos travel in straight lines.

The finding also underscores the promise of 'multi-messenger' astronomy, a nascent field that combines signals from different types of observatory to pin down details of celestial events.

Muon alert

The story began on 22 September 2017, when an electrically charged particle called a muon streaked through the Antarctic ice cap at close to the speed of light. IceCube — an array of more than 5,000 sensors buried in a cubic kilometre’s worth of ice — detected flashes of light that the muon produced in its wake. The particle appeared to emerge from below the detector — an orientation that indicated that it was the decay product of a neutrino that had come from below the horizon. Muons can only travel so far inside matter, whereas neutrinos often pass through the entire planet unimpeded; most of the ones that IceCube detects have crashed with a particle inside Earth to produce a muon (See 'Neutrino Observatory').

Within seconds, a computer cluster at the US National Science Foundation’s Amundsen–Scott South Pole Station, which sits atop Earth's southernmost point, had reconstructed the precise path of the particle and recognized that the muon had come from a highly energetic neutrino; 43 seconds after the event, the station sent an automated alert to a network of astronomers via a satellite link. It tagged the neutrino as IceCube-170922A.

After receiving the alert, Derek Fox, an astrophysicist at Pennsylvania State University in University Park, quickly secured observing time on the X-ray observatory Swift, which orbits Earth. Fox had created the automated alert system two years before, precisely in the hope that researchers could follow up on events such as this one.

He and his team found nine sources of high-energy X-rays close to where the neutrino had come from. Among them was an object called TXS 0506+056. This was a blazar, a galaxy with a supermassive black hole at the centre and a known source of gamma rays. In a blazar, the black hole stirs gas up to temperatures of millions of degrees and shoots it out of its poles in two highly collimated jets, one of which points in the direction of the Solar System. Fox’s team announced its findings to the astronomical community the next day after.

Flare up

In the following days, another team inspected data from the Large Area Telescope (LAT) aboard NASA’s Fermi Gamma-ray Space Telescope. LAT constantly sweeps the sky, and among other things monitors about 2,000 blazars. These objects go through periods of increased activity that can last weeks or months, during which they become unusually bright. “When we looked at the region that IceCube said the neutrino came from, we noticed that this blazar had been flaring more than ever before,” says Regina Caputo, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is Fermi-LAT’s analysis coordinator.

On 28 September, the Fermi-LAT team sent out an alert to reveal this finding. It was at that point that other astronomers got very excited. IceCube has detected about a dozen such high-energy neutrinos each year since it started operating in 2010, but none had been associated with a particular source in the sky. “That’s what made the hair stand at the back of the neck,” Fox says.

Still, the association between the neutrino and the TXS blazar flare could have been a coincidence. To make the case stronger, a team of Fermi-LAT researchers calculated the odds that the flare and the neutrino were related, rather than coming from the same direction in the sky by chance.

“We had to calculate the chance that random neutrinos in the sky come from one of the known gamma-ray sources, and the likelihood that it was flaring at that time,” says Anna Franckowiak, an astroparticle physicist at the German Electron Synchrotron (DESY) in Hamburg who is a member of both IceCube and Fermi-LAT. She and her collaborators found that likelihood to be good, though not at the level of statistical significance required for claiming a discovery in physics. ^{1 , 2}

Evidence hunt

Finding more neutrinos and gamma rays detected during a previous flare from the same blazar would boost the evidence for TXS 0506+056 being the source. In November, IceCube researchers found that the observatory had recorded an excess of neutrinos coming from the same direction in the sky between late 2014 and 2015.

Resconi, who is IceCube’s analysis coordinator, got so excited by the discovery that she got lost while driving to a Nick Cave concert after work. “I ended up in the open countryside. My colleagues now tease me that next time we see a neutrino source, who knows where I will end up.”

Soon though, the researchers realized that this apparent flare did not seem to show up in Fermi-LAT data. “That news came as a wet blanket,” Resconi says. But in a separate study, she and her collaborators found hints of a TXS flare during that period, but with gamma rays of energies that were mostly too high for Fermi-LAT to detect.

A major missing piece of information was the blazar’s distance from Earth, says Simona Paiano of the Astronomical Observatory of Padua in Italy. To measure it, she and her team booked 15 hours of observing time on the world’s largest optical telescope, the 10.4-metre Gran Telescopio Canarias on La Palma, one of Spain’s Canary Islands. They found it to be around 1.15 billion parsecs (3.78 billion light years) away. ³

Together, the data pinpoint the likely source, says Kyle Cranmer, a particle physics and data-analysis expert at New York University, but “the observation isn’t unambiguous”, he says. “More follow-up is needed to conclusively establish blazars as a source of high-energy neutrinos.

Researchers hope that this is only the first of many multi-messenger events of this kind.They are especially looking forward to detecting neutrinos together with gravitational waves. The celebrated collision of two neutron stars that was discovered using gravitational waves in August 2017 should have produced neutrinos as well, but IceCube did not detect any. But if the TXS blazar flares up again, it might be possible to detect more high-energy neutrinos and other kinds of radiation coming from it.