From galaxies far far away!

In a paper that appeared in Science on 22 September (Science 357, 1266–1270 (2017)), the Pierre Auger Collaboration reports observational evidence demonstrating that cosmic rays with energies about a million times greater than that of the protons accelerated in the Large Hadron Collider come from much further away than from our own Galaxy.

Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to where such particles are created. The 50 year-old mystery has been solved using cosmic particles of mean energy of 2 Joules recorded with the largest cosmic-ray observatory ever built, the Pierre Auger Observatory in Argentina.  By studying the distribution of the arrival directions of more than 30000 cosmic rays the Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations, in a direction where the distribution of galaxies is relatively high. It is found that the rate of arrival of cosmic rays is ~6% greater from one half of the sky than from the opposite one, with the excess lying 120 degrees away from the Galactic centre (see Figure). Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down. The direction of the excess points to a broad area of sky rather than to specific sources as even particles as energetic as these are deflected by a few 10s of degrees in the magnetic field of our Galaxy.  The direction, however, cannot be associated with potential sources in the plane or centre of our Galaxy for any realistic configuration of the Galactic magnetic field. Knowledge of the nature of the particles will aid the identification of the sources and work on this problem is targeted in the upgrade of the Auger Observatory to be completed in 2018.

Sky map showing cosmic-ray flux above 8 EeV (colour scale). Galactic center marked with an asterisk
Sky map showing cosmic-ray flux above 8 EeV (colour scale). Galactic center marked with an asterisk

Members of IGFAE contributed directly to this discovery

The Astroparticle Physics group at IGFAE led by Prof. Enrique Zas contributed directly to this important discovery. By analysing inclined showers induced by ultra-high energy cosmic rays that arrive at Earth at large angles with respect to the vertical to the ground, the field of view of the Auger Observatory was extended towards the Northern hemisphere. “This is a long-term project of the whole group and was a crucial step in the determination of the existence of the excess of cosmic rays in the observed direction, since the extended field of view allowed us to compare the rates of cosmic rays from most directions in the sky”, says Inés Valiño who coordinates the work in Auger devoted to the determination of the ultra-high energy cosmic-ray spectrum. The Astroparticle Physics group is composed of five senior members (Jaime Alvarez-Muñiz, Gonzalo Parente, Inés Valiño, Ricardo Vázquez and Enrique Zas) as well as several PhD students (Aida López Casado, Francisco Pedreira and Guillermo Torralba). The group participates in the Pierre Auger Observatory since 2002 when Spain became a full member of the Pierre Auger Collaboration consisting of more than 400 scientists from 20 countries.

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Image information:

Front and in-text sky map image, as measured at the Pierre Auger Observatory ; obtained from IGFAE, extracted from Science 357, 1266–1270 (2017).