Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data
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2016-12-13
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Ezequiel Álvarez et al., Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data, Journal of Cosmology and Astroparticle Physics, Volume 2016 (2016), doi: 10.1088/1475-7516/2016/12/023
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This is the version of the article before peer review or editing, as submitted by an author to Journal of Cosmology and Astroparticle Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/1475-7516/2016/12/023.
This is the version of the article before peer review or editing, as submitted by an author to Journal of Cosmology and Astroparticle Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/1475-7516/2016/12/023.
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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 (gsim 1 GeV) gamma-ray events from the Fermi Large Area Telescope (LAT) and UHECRs (gsim 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 (lesssim 2σ) hint of signal with the correlation function method on scales ~ 1°. Upper limits on the flux of possible power-law gamma-ray sources of UHECRs are derived.