What’s accelerating astrophysical neutrinos to such high energies? This is a decade-old mystery that astronomers believe to have now solved. These tiny neutral particles are thrown out into the universe by blazars, the active supermassive black holes at the center of distant galaxies whose jet of ionized matter is pointed very nearly towards us.

Looking at blazars is like staring down the barrel of a cosmic gun. The light emitted by their relativistic jets easily outshines the light of the rest of their host galaxies. And it's not just light. Several years ago, the IceCube neutrino experiment in Antarctica tracked a high-energy neutrino back to its source, blazar TXS 0506+056.

But that was a single detection, so researchers set out to find if blazars can explain cosmic neutrinos as a whole. They turned once again to IceCube. 

Neutrinos have tiny masses and they have no charge, which allows them to travel through a planet unimpeded. Every second, around 100 trillion neutrinos go through your body. 

Occasionally, one will hit an atom and create a flash of light. Many light detectors placed in a kilometer-sized observatory deep in the Antarctic ice are what makes IceCube so effective at catching these rare events. And, as reported in The Astrophysical Journal Letters, astronomers have now tracked multiple neutrino events to blazars. 

The neutrinos coming from blazars reach energies in the petaelectron volts (PeV), hundreds of times the energy generated in the Large Hadron Collider. The team estimates that there is a less than one in a million chance that these particles don’t come from these PeVatron blazars. 

“The results provide, for the first time, incontrovertible observational evidence that the sub-sample of PeVatron blazars are extragalactic neutrino sources and thus cosmic ray accelerators,” lead author Sara Buson from Julius-Maximilians-Universität (JMU) Würzburg, said in a statement.  

The work brings us a step closer to solving the mystery of the origin of high-energy cosmic rays and where they come from. Blazars look like they could be incredible particle accelerators but more work is necessary to understand how they work.

The work proves once again the importance of multimessenger astronomy, the combination of neutrino and/or gravitational wave observations with regular light telescopes.

“It’s like feeling, hearing and seeing at the same time. You’ll get a much better understanding,” added co-author Clemson University Associate Professor of Physics and Astronomy Marco Ajello. “The same is true in astrophysics because the insight you have from multiple detections of different messengers is much more detailed than you can get from only light.”