Proof that ultra-high-energy cosmic rays come from outside the Milky Way has been published by astrophysicists working on the Pierre Auger Observatory in Argentina. Their measurement has a statistical significance of 5.2σ and appears to settle a decades-old debate about the origins of cosmic rays with energies greater than about 1 EeV (1018 eV).

Cosmic rays are mostly atomic nuclei that bombard Earth from outer space and have energies that range from about 109 eV up to 1020 eV. Because they have electrical charge, cosmic rays are deflected by the magnetic fields that permeate the Milky Way. This process can be likened to the random scattering of light by a thick fog and it tends to destroy all information about where the cosmic rays came from.

As a result, cosmic rays detected on Earth appear to arrive in equal numbers from all directions. This has left astronomers wondering whether the particles are accelerated within the Milky Way, or if they have extragalactic origins.

Small deflections

However, this directional scrambling is not expected to be perfect. It is possible that some directional information could be extracted from measurements on extremely high-energy cosmic rays, because these are not deflected as much by magnetic fields as their lower-energy counterparts.

This has now been confirmed by an international team of researchers that has studied the arrival of more than 30,000 ultra-high-energy cosmic rays with energies greater than 8 EeV.

When a cosmic-ray particle collides with a nucleus in the atmosphere, it creates a shower of billions of particles that rain down on Earth. The Pierre Auger Observatory comprises 1600 Cherenkov particle detectors that are spread over 3000 km2 in Argentina. Multiple detectors see the shower and a careful measurement of the arrival times at each detector gives the direction of the cosmic ray. The energy of the cosmic ray is determined by the intensity of the signals in the Cherenkov detectors.

Fluorescent light

There are also 27 fluorescence telescopes located in four separate places in observatory region. These detect fluorescent light that is emitted when shower particles interact with nitrogen in the atmosphere. This information is used to refine the energy and direction measurements made by the Cherenkov detectors.

The measurements revealed that the arrival rate of ultra-high-energy cosmic rays is about 6% greater in one half of the sky. What is more, the excess lies about 120° away from the centre of the Milky Way – suggesting extra-galactic origins. After correcting its data for the expected bending of these cosmic rays by the magnetic fields of the Milky Way, the team says that the particles appear to be coming from directions in space that have a high density of nearby galaxies.

Exciting result

"I consider this to be one of the most exciting results that we have obtained and one which solves a problem targeted when the observatory was conceived by Jim Cronin and myself over 25 years ago," says Alan Watson of the University of Leeds, who is emeritus spokesperson for the Pierre Auger Observatory.

Because ultra-high-energy cosmic rays are not produced in our galaxy, it is likely that their origins are in galaxies that do not resemble the Milky Way. Watson points to galaxies such as Centaurus A, which appears to contain a supermassive black hole that powers relativistic jets of particles. Watson told Physics World that shock waves in such jets could accelerate nuclei so that they become ultra-high-energy cosmic rays.

The next step for the Pierre Auger Observatory is to get a better understanding of what types of nuclei make up ultra-high-energy cosmic rays. This is the task of the next phase of the observatory, which is called AugerPrime and will run until 2025. This involves covering each Cherenkov detector with a plastic scintillator that is used to detect muons in the cosmic-ray showers. Knowing the muon content will allow scientists to work-out whether the shower was created by a hydrogen or iron nucleus, for example. Different nuclei have different masses and charges, which determine how a cosmic ray is bent by magnetic fields. Having this information could lead to a better understanding of the magnetic fields in the Milky Way – and ultimately pinpoint the sources of ultra-high-energy cosmic rays.

The research is described in Science.