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

Abstract

Context. Very high-energy (VHE) γ-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 γ-ray tail of the observed spectra, could be reproduced in our model and 2) an expected TeV γ-ray spectrum with photon energy >1 TeV of 1ES 0229+200 should be comparable with the 50-h 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.

Particle Acceleration and Synchrotron Self-Compton Emission in Blazar Jets. I. An Application to Quiescent Emission

“…We suggest that the advection is a significant escape mechanism in blazar jet. Since the advection tends to harden the particle distribution, which enhances the high energy components of resulting synchrotron and SSC spectrum from blazar jet, we argue that the model can likely be used to comprehend on the origin of hard spectra, while there are some other interpretations on observed hard spectra from distant blazars (Lefa et al 2011;Zheng & Kang 2013;Cerruti et al 2015;Zheng et al 2016).…”

Evaluating the Classification of Fermi BCUs from the 4FGL Catalog Using Machine Learning

“…This indicates the FSRQ jet has a complex external physical environment, which may lead to complex physical properties in Fermi energy bands (e.g., see Kang et al 2019 for similar discussions). For instance, the photon spectral index of FSRQs is greater than that the index of BL Lacs (see Abdo et al 2010b;Ackermann et al 2011Ackermann et al , 2015 The Fermi-LAT collaboration 2019a,b) which may be the result of the spectrum being superposed with other spectra components for the FSRQs (e.g., Zheng & Kang 2013;Kang et al 2014;Zheng et al 2016;Kang et al 2016;Kang 2017). Therefore, the Fermi band of FSRQs, located at the intersection of both the SSC component and the EC component, may result in more complex observational features (e.g., a hard spectrum, Abdo et al 2009Abdo et al , 2010bAckermann et al 2011Ackermann et al , 2015.…”

The local-filament pattern in the anomalous transparency of the Universe for energetic gamma rays

“…Cascades can develop in the cosmic background fields and radiation, starting either from a VHE photon [26] or from an accompanying UHE proton emitted by the same source [27] (see also Refs. [28][29][30][31][32][33]); what we detect at the Earth may be secondary particles produced nearby. Conservation means that the photon converts near the source to some particle, which does not produce pairs on EBL, then this particle travels unattenuated and reconverts back to a photon close to us.…”