Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic-ray emission

Abstract

We present a search for high-energy $\gamma$-ray emission from 566 Active Galactic Nuclei at redshift $z>0.2$, from the 2WHSP catalog of high-synchrotron peaked BL Lac objects with eight years of Fermi-LAT data. We focus on a redshift range where electromagnetic cascade emission induced by ultra-high-energy cosmic rays can be distinguished from leptonic emission based on the spectral properties of the sources. Our analysis leads to the detection of 160 sources above $\approx$ $5\sigma$ (TS $\ge 25$) in the 1 - 300 GeV energy range. By discriminating significant sources based on their $\gamma$-ray fluxes, variability properties, and photon index in the Fermi-LAT energy range, and modeling the expected hadronic signal in the TeV regime, we select a list of promising sources as potential candidate ultra-high-energy cosmic-ray emitters for follow-up observations by Imaging Atmospheric Cherenkov Telescopes.

15 pages, 9 figures, 2 tables, submitted to MNRAS

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• Predictions of TeV emission for a set of hard BL Lac objects