After weeks of speculation, today the IceCube neutrino observatory has revealed their big news. After more than a century of debate about the origin of high energy cosmic rays, the IceCube team together with astronomers around the world have managed to pinpoint the source of at least some high energy cosmic rays as being a very active and violent distant galaxy known as a blazar. (For those interested in the technical details, the two research papers can be found here and here)

Cosmic rays were first detected in 1912 by Victor Hess, although there is some debate since other scientists had measured cosmic rays three years earlier but did not recognize them as coming from beyond the Earth's atmosphere. They are high energy particles (usually protons, antiprotons, or small atomic nuclei) that can travel through deep space for millions or even billions of years before colliding with the atmosphere and fragmenting into a shower of lighter, lower energy particles. It is this fragmentary nature that makes them difficult to study, since the biggest detectors will still only record a fraction of the energy and particles contained in the original cosmic ray. What we do know is that they are more energetic than anything created on Earth or even in our neighbourhood of the Milky Way galaxy. 

Over the years, many theories have been put forth on what creates these high energy particles and where they are coming from. There are very few processes in the Universe that can generated large numbers of very high energy particles, and even fewer methods of tracing the cosmic rays back to their origin point. One possibility is that they are generated in supernovae, the violent and energetic explosions that mark the end of a star's life cycle. Data from the Fermi space telescope in 2013 indicated that a significant fraction of cosmic rays detected on Earth are generated in supernovae, but even these are are not energetic enough to explain the highest energy particles that have been detected.

Another possibility that has been considered in active galactic nuclei. Most galaxies, including our own, are relatively tame. There is a massive black hole at the center of the Milky Way, but the stars and other objects in the galactic core tend to orbit it without much interaction. However in some galaxies, the black holes are far more active. The black holes are believed to be absorbing the galaxies around them, and the strong gravitational fields and strong magnetic fields are causing very high energy reactions to occur throughout their accretion disks. These reactions can accelerate particles to speeds at close to the speed of light, and energies that are many orders of magnitude greater than anything produced in terrestrial particle accelerators, as well as generating high energy photons in several wavelengths.

All active galactic nuclei generate beams of high energy particles along their axis of rotation. For a small fraction of these black holes, that axis points to our Solar System, meaning that we are in the path of these high energy jets. The high energy electromagnetic waves generated in these galactic cores, and hitting the Earth, lead to these particle galaxies being dubbed blazars.

The discovery by IceCube and confirmed by others is that one of these blazars is generating high energy cosmic rays. This particular blazar is known as TXS 0506+056, and is located in the direction of the Orion constellation, and is nearly four billion light years from the Earth. The obvious question at this point is, why when we are interested in cosmic rays - which are heavy, charged particles - would we use a detector that studies neutrinos - electrically neutral, nearly massless particles?

The reason is that neutrinos can be tracked. The same properties that makes them very difficult to detect makes them a perfect marker for the origin of cosmic rays.

The high energy protons and antiprotons that are generated and emitted by the blazars follow a circuitous route through space. Most objects in space, including galaxies and other large scale structures, generate electromagnetic fields. And some of these magnetic fields are quite strong and have quite a large range of effect. As the cosmic ray passes the edge of a galaxy, its path gets bent and starts moving in a different direction. Some cosmic rays will also interact with the cosmic microwave background or with extragalactic gamma rays, which also cause them to change direction. The end result is that the once narrow jet emitted by the blazar will spread out making it impossible to determine which direction they originated from.

However neutrinos have no interaction with electromagnetic fields. The same reason that they are difficult to detect in particle physics experiments - their lack of interactions with electromagnetic fields and only short range weak nuclear interactions - make them immune from these cosmic disruptions. If you can detect a neutrino and determine its direction of travel, you can be fairly certain of the direction in space that it came from originally. 

And so that is what was done in the latest experiment. The IceCube observatory waited for a burst of high energy neutrinos, and determined the direction that they came from. This happened on September 22, 2017, and the IceCube team immediately sent the coordinates for the neutrino burst to several gamma-ray telescopes, including the Fermi Space Telescope which is in orbit, and the MAGIC telescope in the Canary Islands. The gamma-ray telescopes immediately focused on that region of space, and detected the gamma-ray and cosmic ray bursts, which were then confirmed by a number of other telescopes that were able to detect the same burst in the optical range, as well as in radio telescopes and x-ray telescopes.

And all of the astronomers at all of the telescopes were able to match up the origin of the burst as being the TXS 0506+056 blazar, marking the first clear evidence that it is the source of the high energy cosmic rays. 

In the end it was a very interesting result, solving a century old mystery, and proving once more the value of multimessenger astronomer and collaboration between very different experiments. We know where the cosmic rays came from!