Differences Between the Pierre Auger Observatory and Telescope Array Spectra: Systematic Effects or Indication of a Local Source of Ultra-High-Energy Cosmic Rays?

TL;DR

This study compares the energy spectra of ultra-high-energy cosmic rays observed by the Pierre Auger Observatory and Telescope Array, exploring whether differences are due to systematic effects or a local astrophysical source.

Contribution

It introduces a joint fit model considering both systematic uncertainties and local sources, providing insights into the origin of spectral differences between the two observatories.

Findings

Both systematic effects and local sources explain the spectral differences equally well.

A local source within 26 Mpc emitting silicon-dominated cosmic rays best fits the data.

The local source hypothesis may account for TA anisotropy and spectral variations across declination bands.

Abstract

The Pierre Auger Observatory (PAO) and Telescope Array (TA) collaborations report significant differences in the observed energy spectra of ultra-high-energy cosmic rays (UHECRs) above 30~EeV. In this work we present a joint fit of TA and PAO data using the rigidity-dependent maximum energy model, including a full marginalization over all relevant parameters. We test two possible scenarios to explain these differences. One is that they are due to complex energy-dependent experimental systematics; the other is the presence of a local astrophysical source in the Northern Hemisphere, which is only visible by the TA experiment. We show that the astrophysical and systematic scenarios improve the explanation of the data equally well, compared to the scenario where both experiments observe the same UHECR flux from a cosmological source distribution and have energy-independent systematics. We test different mass compositions emitted from the local source and conclude that the data are best described by a source lying at a distance below 26~Mpc that emits cosmic rays dominated by the silicon mass group. We also discuss possible source candidates, and the possible role of the putative local UHECR source in the observed TA anisotropy and in the differences in TA spectral data from different declination bands.