Constraining astrophysical sources of intermediate-mass Ultra-high energy cosmic rays (UHECRs)

Abstract

Constraining the astrophysical source populations of Ultra High Energy Cosmic Rays (UHECRs) is difficult because UHECRs are deflected by the Galactic Magnetic Field (GMF). Recent interpretations of cosmic-ray-produced air showers with LHC-tuned hadronic interaction models suggest a gradual increase in the mean mass of UHECRs with energy. The decades-old view of UHECRs being a mix of hydrogen and iron (with relative composition varying with energy) is now expanding to also consider intermediate nuclear compositions. Notably, while hydrogen and iron UHECRs have expected mean free paths of ∼ 100Mpc, intermediate composition UHECRs have mean free paths of only 10s of Mpc. Monte-Carlo simulations of H, C, N, and O composition UHECR tracks in 8 proposed GMF models can be used to estimate deflections suffered by the UHECRs detected by the Pierre Auger Observatory and the Telescope Array. These deflections can be used to identify sub-samples of 'least-deflected' UHECRs relatively independent of the GMF model assumed. The distribution of the GMF-deflection-corrected arrival directions of this 'least deflected' sample can be correlated with astrophysical catalogs to best constrain whether a substantial population of UHECRs are of intermediate composition and originate in a specific type of astrophysical source in the very local universe. Sotomayor et al. 2022 found a strong association between the (GMF-deflection-corrected) arrival directions of 'least deflected' UHECRs with nearby (D ≤ 20 Mpc) galaxies when considering an oxygen UHECR composition. In this poster we present a continuation of the [1] analysis, but now using three high-abundance intermediate mass species (C, N, and O), and now weighting astrophysical samples by the fluxes, luminosities, and other properties (e.g., galaxy mass, star formation rate, radio and X-ray emission) of their galaxies.