Cosmic rays with an extragalactic origin!

It is a 50-year old debate which has now been settled: the highest-energy cosmic rays do not originate from the Milky Way, but have been fired from galaxies located tens, or even hundreds of millions of light-years away. This discovery has just been announced by an international collaboration, including researchers from the Institute of Nuclear Physics at Orsay (CNRS [National Centre for Scientific Research]/Université Paris-Sud).

Cosmic rays are atomic nuclei which pass through our universe at a speed close to that of light. The low-energy ones come from the sun or our galaxy, but the origin of the highest-energy particles has been debated since their discovery 50 years ago: are they from the Milky Way or distant extragalactic objects? 

The question has just been settled by studying 30 000 cosmic particles with an energy a million times greater than that of protons accelerated at the LHC . They were detected between 2004 and 2016 with the largest cosmic ray observatory ever built, the Pierre Auger Observatory, in Argentina.

Study of the arrival directions of these particles shows that at these energies, the flux of cosmic rays coming from a sky region located 120 degrees from the galactic centre, is approximately 6 % higher than if the flux were perfectly uniform. This direction cannot be linked with potential sources in the galactic plane or at its centre.

This discovery clearly indicates an extragalactic origin for these cosmic particles, as the pattern observed in the sky is no mere coincidence, with a probability of one in five million. However, this study does not yet precisely locate the sources. Indeed, the region brightest in cosmic rays, covers a large part of the sky, where the number of galaxies is relatively high. Furthermore, the Milky Way's magnetic field deflects the trajectories of these charged particles and covers their tracks.  

Research is currently being conducted on a much larger collection of ultra-high-energy cosmic rays and could provide some answers. At the same time, an upgrade programme is underway at the Pierre Auger Observatory and should make it possible to more clearly identify these sources.

The Pierre Auger Observatory
400 scientists from 18 countries are involved in the Pierre Auger Collaboration, which develops and runs the observatory with the same name. French laboratories contributing to it are:
- the Institute of Nuclear Physics at Orsay (CNRS/Université Paris-Sud);
- the Nuclear and High-Energy Physics Laboratory (CNRS/UPMC/Université Paris Diderot);
- the Laboratory of Sub-Atomic Physics and Cosmology (CNRS/Université Grenoble Alpes/ Grenoble INP).

One of the 1 600 detectors at the Pierre Auger Observatory  
The flux of very high-energy cosmic rays (over 2 joules) is approximately 1 per square kilometre per year. When these rays collide with molecules in the upper atmosphere, they create a cascade of more than 10 billion secondary particles, called an air shower, which can extend over more than 40 square kilometres when it reaches the ground. The Pierre Auger Observatory detects some of these secondary particles (electrons, photons and muons) through an array of 1 600 detectors – tanks of pure water spaced 1.5 kilometres apart which cover an area of 3 000 square kilometres in the Argentine pampas (or slightly more than the size of Luxembourg). By comparing the arrival times of the particles in the different detectors, it can be determined where the cosmic ray that produced the air shower comes from.

The Pierre Auger Observatory in Argentina is the world’s largest detector of cosmic rays. It bears the name of the French physicist who first observed air showers in 1938.

Reference
Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8 × 1018 eV, The Pierre Auger Collaboration. Science, 22 September 2017. DOI: 10.1126/science.aan4338