Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data

TL;DR

This study searches for correlations between ultra high-energy cosmic rays and high-energy gamma-ray data from Fermi-LAT, finding no significant associations but setting upper limits on gamma-ray fluxes related to UHECR sources.

Contribution

It provides a comprehensive multi-method analysis of UHECR and gamma-ray correlations, introducing new upper limits on gamma-ray fluxes from potential UHECR sources.

Findings

No significant correlation detected in the analyses.

A weak hint of correlation at ~1° scale with the correlation function method.

Upper limits established on gamma-ray fluxes from UHECR sources.

Abstract

The astrophysical sources responsible for ultra high-energy cosmic rays (UHECRs) continue to be one of the most intriguing mysteries in astrophysics. We present a comprehensive search for correlations between high-energy ($\gtrapprox 1$ GeV) gamma-ray events from the Fermi Large Area Telescope (LAT) and UHECRs ($\gtrapprox 60$ EeV) detected by the Telescope Array and the Pierre Auger Observatory. We perform two separate searches. First, we conduct a standard cross-correlation analysis between the arrival directions of 148 UHECRs and 360 gamma-ray sources in the Second Catalog of Hard Fermi-LAT sources (2FHL). Second, we search for a possible correlation between UHECR directions and unresolved Fermi-LAT gamma-ray emission. For the latter, we use three different methods: a stacking technique with both a model-dependent and model-independent background estimate, and a cross-correlation function analysis. We also test for statistically significant excesses in gamma rays from signal regions centered on Cen A and the Telescope Array hotspot. No significant correlation is found in any of the analyses performed, except a weak ($\lessapprox 2\sigma$) hint of signal with the correlation function method on scales $\sim 1^\circ$. Upper limits on the flux of possible power-law gamma-ray sources of UHECRs are derived.