Bethe-Heitler cascades as a plausible origin of hard spectra in distant TeV blazars

TL;DR

This paper proposes a model where Bethe-Heitler cascades explain the hard gamma-ray spectra observed in distant TeV blazars, combining internal SSC and external cascade components, and demonstrates its consistency with observed spectra and future observatory sensitivities.

Contribution

The paper introduces a novel model combining internal SSC and external cascade emissions to explain hard spectra in distant TeV blazars, supported by application to observed data.

Findings

The model reproduces the observed spectrum of 1ES 0229+200.

Predicted TeV spectra are within CTA and LHAASO sensitivity ranges.

Strong evidence for Bethe-Heitler cascades can be obtained from future VHE observations.

Abstract

Context. Very high-energy (VHE) $\gamma$-ray measurements of distant TeV blazars can be nicely explained by TeV spectra induced by ultra high-energy cosmic rays. Aims. We develop a model for a plausible origin of hard spectra in distant TeV blazars. Methods. In the model, the TeV emission in distant TeV blazars is dominated by two mixed components. The first is the internal component with the photon energy around 1 TeV produced by inverse Compton scattering of the relativistic electrons on the synchrotron photons (SSC) with a correction for extragalactic background light absorbtion and the other is the external component with the photon energy more than 1 TeV produced by the cascade emission from high-energy protons propagating through intergalactic space. Results. Assuming suitable model parameters, we apply the model to observed spectra of distant TeV blazars of 1ES 0229+200. Our results show that 1) the observed spectrum properties of 1ES 0229+200, especially the TeV $\gamma$-ray tail of the observed spectra, could be reproduced in our model and 2) an expected TeV $\gamma$-ray spectrum with photon energy $>$1 TeV of 1ES 0229+200 should be comparable with the 50-hour sensitivity goal of the Cherenkov Telescope Array (CTA) and the differential sensitivity curve for the one-year observation with the Large High Altitude Air Shower Observatory (LHAASO). Conclusions. We argue that strong evidence for the Bethe-Heitler cascades along the line of sight as a plausible origin of hard spectra in distant TeV blazars could be obtained from VHE observations with CTA, LHAASO, HAWC, and HiSCORE.