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Description
The Pierre Auger Observatory is the world's largest detector for extensive air showers initiated by cosmic rays with energies above $3\times10^{17}\,\mathrm{eV}$. The main objectives of the Observatory are the measurement of the energy spectrum up to the highest energies, the determination of the mass composition and the search for the origin of the ultra-high energy particles. The manifold studies profit from the hybrid detector design, featuring a surface detector array of 1660 water-Cherenkov stations spread over an area of $3000\,\mathrm{km}^2$ as well as a fluorescence detector comprised of 27 telescopes monitoring the atmosphere. With the data recorded during the last decade, the suppression of the cosmic ray flux at energies above $4\times10^{19}\,\mathrm{eV}$ was clearly established. The question whether the suppression originates solely from the propagation of the cosmic rays or rather from the acceleration limitations of their sources has yet to be answered. This question is closely related to the question of the sources of cosmic rays. The Observatory recently measured a dipole anisotropy, whose orientation indicates an extragalactic origin for particles with energies above $8\times10^{18}\,\mathrm{eV}$. Anisotropy studies on smaller scales necessitate a precise knowledge of the mass composition at the highest energies. The current upgrade of the Observatory to AugerPrime increases the mass sensitivity and will help to elucidate the origin of the ultra-high energy cosmic rays.