R. Conceição

Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above 6 x 10(19) electron volts and the positions of active galactic nuclei (AGN) lying within approximately 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest-energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.

The energy spectrum of cosmic rays above $2.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\mathrm{eV}$, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index $\ensuremath{\gamma}$ of the particle flux, $J\ensuremath{\propto}{E}^{\ensuremath{-}\ensuremath{\gamma}}$, at energies between $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\mathrm{eV}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\mathrm{eV}$ is $2.69\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$, steepening to $4.2\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$ at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.

We describe the measurement of the depth of maximum, ${X}_{\mathrm{max}}$, of the longitudinal development of air showers induced by cosmic rays. Almost 4000 events above ${10}^{18}\text{ }\text{ }\mathrm{eV}$ observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of $({106}_{\ensuremath{-}21}^{+35})\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{2}/\mathrm{\text{decade}}$ below ${10}^{18.24\ifmmode\pm\else\textpm\fi{}0.05}\text{ }\text{ }\mathrm{eV}$, and $(24\ifmmode\pm\else\textpm\fi{}3)\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{2}/\mathrm{\text{decade}}$ above this energy. The measured shower-to-shower fluctuations decrease from about 55 to $26\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{2}$. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.

High-energy particles are extragalactic Cosmic rays are high-energy particles arriving from space; some have energies far beyond those that human-made particle accelerators can achieve. The sources of higher-energy cosmic rays remain under debate, although we know that lower-energy cosmic rays come from the solar wind. The Pierre Auger Collaboration reports the observation of thousands of cosmic rays with ultrahigh energies of several exa–electron volts (about a Joule per particle), arriving in a slightly dipolar distribution (see the Perspective by Gallagher and Halzen). The direction of the rays indicates that the particles originated in other galaxies and not from nearby sources within our own Milky Way Galaxy. Science , this issue p. 1266 ; see also p. 1240