Constraining models for the origin of ultra-high-energy cosmic rays with spectrum, composition, and arrival direction data measured at the Pierre Auger Observatory
July 25, 2023
The distribution of cosmic-ray arrival directions shows a better agreement with models in which
a fraction of the flux is associated with catalogs of nearby source candidates, such as starburst
galaxies, than with isotropy. To investigate this further, we use a novel approach, fitting simultaneously
the energy spectrum, distributions of shower maxima, and arrival directions at the highest
energies measured with the Pierre Auger Observatory. The astrophysical model consists of homogeneously
distributed background sources as well as an adaptable contribution from nearby
source candidates. Propagation effects and a rigidity-dependent magnetic field blurring are taken
into account, producing a rising level of anisotropy with the energy. We demonstrate that a model
containing a flux fraction of around 20% from the starburst galaxy catalog at 40 EeV, with a hard,
nitrogen-dominated injection spectrum, provides a good description of the data. By investigating
a scenario with Cen A as a single source in combination with the homogeneous background, we
show that this region of the sky provides the dominant part to the observed anisotropy signal.
Models based on jetted active galactic nuclei whose cosmic-ray flux scales with the gamma-ray
emission are disfavored. The modeled energy evolution of the arrival directions, the spectra of
individual sources, as well as the statistical significance of the results, including the influence of
experimental systematic effects, will be discussed in this contribution.
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