Ultrahigh-energy Cosmic-Ray Interactions as the Origin of Very High-energy γ-Rays from BL Lacertae Objects

Abstract

We explain the observed multiwavelength photon spectrum of a number of BL Lac objects detected at very high energy (VHE, E 30 GeV), using a lepto-hadronic emission model. The one-zone leptonic emission is employed to fit the synchrotron peak. Subsequently, the SSC spectrum is calculated, such that it extends up to the highest energy possible for the jet parameters considered. The data points beyond this energy, and also in the entire VHE range are well explained using a hadronic emission model. The ultrahigh-energy cosmic rays (UHECRs, E 0.1 EeV) escaping from the source interact with the extragalactic background light (EBL) during propagation over cosmological distances to initiate electromagnetic cascade down to ∼ 1 GeV energies. The resulting photon spectrum peaks at ∼ 1 TeV energies. We consider a random turbulent extragalactic magnetic field (EGMF) with a Kolmogorov power spectrum to find the survival rate of UHECRs within 0.1 degrees of the direction of propagation in which the observer is situated. We restrict ourselves to an RMS value of EGMF, B rms ∼ 10 −5 nG, for a significant contribution to the photon spectral energy distribution (SED) from UHECR interactions. We found that UHECR interactions on the EBL and secondary cascade emission can fit gamma-ray data from the BL Lacs we considered at the highest energies. The required luminosity in UHECRs and corresponding jet power are below the Eddington luminosities of the super-massive black holes in these BL Lacs.

Cosmogenic gamma-ray and neutrino fluxes from blazars associated with IceCube events

“…Thus, there exists a degeneracy in explaining the blazar SED, which can be lifted by future multimessenger observations. Ultra-high-energy (UHE) protons, if accelerated, can escape the jet when the escape timescale is shorter than the acceleration time (see, e.g., Das et al 2020). These UHECRs can interact with photons from the cosmic microwave background (CMB) and extragalactic background light (EBL) to produce secondary neutrinos and γ rays with a hard spectrum along the line-of-sight Kalashev et al 2013;Das et al 2021).…”

Cosmic Ray Indirect Rapporteur Report (37th ICRC )

“…• The contribution of Das [51] showed that in principle extreme BL Lac objects (those with energy peaks at very high energies with respect to the rest of the BL Lac population) can be sources of UHECRs and that the UHECR contribution can help fit the source spectra nicely. But in general one does not expect neutrinos from these sources (there are too few).…”

A lepto-hadronic model of gamma-ray signal from extreme blazars

“…For detailed paramter values of all the sources considered in the study, please see Ref. [8]. interactions with synchrotron and IC photons inside the jet are compared with the escape timescale esc and the acceleration timescale acc (cf.…”

“…Here we present the results for only 1ES 0229+200. Please refer to [8] for a detailed and fuller analysis.…”

“…Figure1: Left: Distribution of propagated UHECRs as a function of deflection angle in a random turbulent magnetic field. Right: Timescale of photohadronic interactions inside the jet with synchrotron and IC photons, along with the acceleration and escape timescale; for one of the sources considered in the study[8].…”