Indication of anisotropy in arrival directions of ultra-high-energy cosmic rays through comparison to the flux pattern of extragalactic gamma-ray sources

Abstract: A new analysis of the dataset from the Pierre Auger Observatory provides evidence for anisotropy in the arrival directions of ultra-high-energy cosmic rays on an intermediate angular scale, which is indicative of excess arrivals from strong, nearby sources. The data consist of 5514 events above 20 EeV with zenith angles up to 80 deg recorded before 2017 April 30. Sky models have been created for two distinct populations of extragalactic gamma-ray emitters: active galactic nuclei from the second catalog of hard Fermi-LAT sources (2FHL) and starburst galaxies from a sample that was examined with Fermi-LAT. Flux-limited samples, which include all types of galaxies from the Swift-BAT and 2MASS surveys, have been investigated for comparison. The sky model of cosmic-ray density constructed using each catalog has two free parameters, the fraction of events correlating with astrophysical objects and an angular scale characterizing the clustering of cosmic rays around extragalactic sources. A maximum-likelihood ratio test is used to evaluate the best values of these parameters and to quantify the strength of each model by contrast with isotropy. It is found that the starburst model fits the data better than the hypothesis of isotropy with a statistical significance of 4.0 sigma, the highest value of the test statistic being for energies above 39 EeV. The three alternative models are favored against isotropy with 2.7-3.2 sigma significance. The origin of the indicated deviation from isotropy is examined and prospects for more sensitive future studies are discussed.

Anisotropy in Ultra-High-Energy Cosmic Ray Arrival Directions

The study conducted by the Pierre Auger Collaboration investigates the anisotropy in arrival directions of ultra-high-energy cosmic rays (UHECRs), offering insights into potential sources of these enigmatic particles. The research scrutinizes an extensive dataset comprising 5514 UHECR events with energies exceeding 20 EeV, collected by the Pierre Auger Observatory up to April 2017.

Methodology and Data Analysis

The focal point of this investigation is the correlation between UHECRs and theoretically predicted anisotropic patterns emanating from extragalactic gamma-ray sources. Two principal categories of sources are interrogated: active galactic nuclei (AGNs) mapped from the 2FHL catalog, and starburst galaxies (SBGs) evaluated through Fermi-LAT data. Additionally, the study cross-references flux-limited samples from Swift-BAT and 2MASS surveys, encapsulating a diverse array of galaxies across different energy bands.

To assess anisotropy, the research employs a sky model of cosmic-ray density influenced by the catalogs mentioned above. The model variables include the fraction of events related to astrophysical objects and an angular scale characterizing cosmic-ray clustering. A maximum-likelihood ratio test is utilized to ascertain the most accurate model parameters, measuring the model's strength against isotropy.

Results

The study reveals that the starburst galaxy model exhibits the highest statistical correlation with the observed data, achieving a deviation from isotropy with a significance of 4.0σ for cosmic-ray energies above 39 EeV. Conversely, models based on AGNs and other flux samples also exhibit anisotropic tendencies, albeit with reduced significance between 2.7σ and 3.2σ.

Interestingly, the analysis suggests that while starburst galaxies offer compelling candidates for local UHECR contributors, AGNs and other astrophysical catalogs remain plausible candidates, especially when considering the potential deflection effects caused by intervening magnetic fields. The implications suggest a blend of contributing sources, with starburst galaxies playing a prominent role at particular energy thresholds.

Implications and Prospects

This study is transformational in enhancing the understanding of UHECR sources and their distribution across the sky. The observed anisotropy at intermediate angular scales enriches the narrative around the probable origin of cosmic rays, with local structures like starburst galaxies emerging as major contributors.

Moreover, the results pave the way for further refined studies incorporating magnetic field models to account for cosmic-ray deflection and potentially leveraging multi-observatory datasets, such as those from the Northern Hemisphere Telescope Array, to bolster coverage and precision.

In summary, the Pierre Auger Collaboration's findings underscore the importance of both empirical and theoretical pursuits in unraveling the cosmic-ray enigma and furnish substantial groundwork for future explorations into the high-energy universe.