The topic of high-energy cosmic rays has recently attracted significant attention. While the AGASA and HiRes Observatories have closed after many years of successful operation, the Pierre Auger Observatory began taking data in January 2004 and the first results have been reported. Plans for the next generation of instruments are in hand: funding is now being sought for the northern phase of the Auger Observatory and plans for a space detector, JEM-EUSO, to be launched in 2013–14 are well advanced with the long-term target of a dedicated satellite for the 2020s. It therefore seemed an appropriate time to make a collection of outstanding and original research articles from the leading experimental groups and from some of the theorists who seek to interpret the hard-won data and to speculate on the origin of the highest energy cosmic rays.

This focus issue in New Journal of Physics on the topic of high energy cosmic rays, contains a comprehensive account of the work of the Yakutsk group (A A Ivanov, S P Knurenko and I Ye Sleptsov) who have used Cerenkov radiation produced by shower particles in the air to provide the basis for energy calibration. This technique contrasts with that of detecting fluorescence radiation from space that is proposed for the JEM-EUSO instrument to be placed on the International Space Station in 2013, described by Y Takahashi. Supplementing this is an article by A Santangelo and A Petrolini describing the scientific goals, requirements and main instrument features of the Super Extreme Universe Space Observatory mission (S-EUSO).

The use of fluorescence light to measure energies was the key component of the HiRes instrument and is also used extensively by the Pierre Auger Collaboration so an article, by F Arqueros, F Blanco and J Rosado, summarizing the properties of fluorescence emission, still not fully understood, is timely. M Nagano, one of the architects of the AGASA Observatory, has provided an overview of the experimental situation with regard to the energy spectrum of the highest energy cosmic rays.

The remaining contributions are of a more theoretical nature and discuss propagation (T Stanev), the time structure of multi-messenger signals (G H W Sigl), ultra-high energy cosmic ray production near black holes (A Yu Neronov, D V Semikoz and I I Tkachev), production in jets associated with black holes (C D Dermer, S Razzaque, J Finke and A Atoyan) and emission from a specific object, Cen A (M Kachelriess, S S Ostapchenko and R Tomas). Additionally the potential of high energy cosmic rays to give information about features of hadronic interactions, specifically the cross-section for p–air collisions, is discussed in the paper by R Ulrich et al.

We thank all our authors most sincerely for their efforts and Tim Smith and his editorial team for their hard work. We believe that this collection of articles will be of great value to workers in the field: further contributions to this focus issue will be published during the course of 2009.

Focus on High Energy Cosmic Rays Contents

The cosmic ray energy spectrum as measured using the Pierre Auger Observatory
Giorgio Matthiae

The northern site of the Pierre Auger Observatory
Johannes Blümer and the Pierre Auger Collaboration

Searching for new physics with ultrahigh energy cosmic rays
Floyd W Stecker and Sean T Scully

On the measurement of the proton–air cross section using air shower data
R Ulrich, J Blümer, R Engel, F Schüssler and M Unger

High energy radiation from Centaurus A
M Kachelrieß, S Ostapchenko and R Tomàs

Ultra-high-energy cosmic rays from black hole jets of radio galaxies
C D Dermer, S Razzaque, J D Finke and A Atoyan

Ultra-high energy cosmic ray production in the polar cap regions of black hole magnetospheres
A Yu Neronov, D V Semikoz and I I Tkachev

Time structure and multi-messenger signatures of ultra-high energy cosmic ray sources
Günter Sigl

Propagation of ultrahigh-energy cosmic rays
Todor Stanev

Search for the end of the energy spectrum of primary cosmic rays
M Nagano

Analysis of the fluorescence emission from atmospheric nitrogen by electron excitation, and its application to fluorescence telescopes
F Arqueros, F Blanco and J Rosado

Observing ultra-high-energy cosmic particles from space: S-EUSO, the Super-Extreme Universe Space Observatory Mission
A Santangelo and A Petrolini

The JEM-EUSO mission
Yoshiyuki Takahashi and the JEM-EUSO Collaboration

Measuring extensive air showers with Cherenkov light detectors of the Yakutsk array: the energy spectrum of cosmic rays
A A Ivanov, S P Knurenko and I Ye Sleptsov