Victor Hess discovered cosmic rays back in 1912, but it proved incredibly difficult to identify the astrophysical source of these bullets. The obstacle to progress was the fact that charged cosmic rays – whether protons or atomic nuclei – don’t follow a straight-line path from source to Earth. Instead, the paths get bent and twisted by magnetic fields in space. Just because a cosmic ray appears to come from a particular direction of sky doesn’t mean it really did come from that direction. It seems to be an insurmountable problem.
But we are now in the era of multi-messenger astronomy! And that allows astronomers to answer questions that once seemed impossible.
The key to unlocking the cosmic ray mystery is that the violent events that generate high-energy charged particles will also generate neutrinos. And neutrinos do follow a straight-line path from source to Earth: because they interact solely via the weak force their paths aren’t bent by magnetic fields, and they don’t get absorbed or scattered by intervening matter. Neutrinos can act as tracers of high-energy cosmic rays. Of course, the same properties that make them useful tracers also make them incredibly difficult to detect: indeed until recently, apart from a diffuse neutrino background, astronomers had managed to confirm only two astrophysical sources of neutrinos: the Sun and SN1987A (the latter being a relatively close supernova). The IceCube observatory, however, now has good evidence for a third source: TXS 0506+056. And this might have solved the mystery of high-energy cosmic rays.
In September 2017, IceCube – a neutrino telescope consisting of detectors buried in a cubic kilometer of South Pole ice – spotted a neutrino with an energy of 290TeV. (That’s 40 times more energetic than the particles accelerated by the LHC.) Astronomers could trace it back to a source in the direction of Orion. IceCube sent out an alert to observatories around the world, and several of them – Fermi, MAGIC, HAWC and others – detected an increase in gamma-ray activity from the same patch of sky. The culprit was TXS 0506+056 – a blazar that’s about four billion light years away.
A blazar is an active galactic nucleus – the compact central region of a galaxy where a supermassive black hole sucks material onto an accretion disk and spews out radiation in two opposing relativistic jets. When we see a blazar, we just happen to be looking directly down one of the jets. It’s quite a thought: four billion years ago the central black hole of a galaxy hurled neutrinos and charged particles and gamma radiation towards Earth. Magnetic fields steered the charged particles away from us. But the neutrinos and gamma rays made it to Earth. And, in September 2017, IceCube detected one of those neutrinos.
The “Case of the High-Energy Cosmic Rays” isn’t entirely closed. Astronomers would want to see more examples before they can be sure that active galactic nuclei are the source. But the observation is very, very suggestive.
And, as with all else in science, the answer to one question raises others: Can other objects besides active galactic nuclei produce high-energy cosmic rays? What is the exact mechanism whereby these particles are produced? And what is the source of the most powerful cosmic rays – are blazars responsible for them too? Now that we are in the age of multi-messenger astronomy, an age in which we can observe astrophysical events not only across the entire electromagnetic spectrum but also with gravitational wave telescopes and neutrino telescopes … well, the answers might start to come more quickly.