Disentangling Hadronic and Leptonic Cascade Scenarios from the Very-High-Energy Gamma-Ray Emission of Distant Hard-Spectrum Blazars

TL;DR

This paper explores whether very-high-energy gamma-ray emissions from distant blazars originate from source emissions or cosmic-ray induced cascades, proposing CTA observations as a means to distinguish these scenarios and identify cosmic-ray origins.

Contribution

It introduces a method to differentiate gamma-ray emission scenarios from blazars using CTA measurements, emphasizing the importance of redshift accuracy for understanding cosmic-ray origins.

Findings

CTA can distinguish emission scenarios by measuring flux above 500 GeV.

Preliminary H.E.S.S. data suggest cosmic-ray induced cascades for KUV 00311-1938.

Redshift measurements are crucial for interpreting gamma-ray origins.

Abstract

Recent data from the \emph{Fermi} Large Area Telescope have revealed about a dozen distant hard-spectrum blazars that have very-high-energy (VHE; $\gtrsim 100$ GeV) photons associated with them, but most of them have not yet been detected by imaging atmospheric Cherenkov telescopes. Most of these high-energy gamma-ray spectra, like those of other extreme high-frequency peaked BL Lac objects, can be well explained either by gamma rays emitted at the source or by cascades induced by ultra-high-energy cosmic rays, as we show specifically for KUV 00311$-$1938. We consider the prospects for detection of the VHE sources by the planned Cherenkov Telescope Array (CTA) and show how it can distinguish the two scenarios by measuring the integrated flux above $\sim 500$ GeV (depending on source redshift) for several luminous sources with $z \lesssim 1$ in the sample. Strong evidence for the origin of ultra-high-energy cosmic rays could be obtained from VHE observations with CTA. Depending on redshift, if the often quoted redshift of KUV 00311-1938 ($z = 0.61$) is believed, preliminary H.E.S.S. data favor cascades induced by ultra-high-energy cosmic rays. Accurate redshift measurements of hard-spectrum blazars are essential for this study.