The Impact of Plasma Instabilities on the Spectra of TeV Blazars

Abstract

Relativistic jets from blazars are known to be sources of very-high-energy gamma rays (VHE-GRs). During their propagation in the intergalactic space, VHEGRs interact with pervasive cosmological photon fields such as the extragalactic background light (EBL) and the cosmic microwave background (CMB), producing electron-positron pairs. These pairs can upscatter CMB/EBL photons to high energies via inverse Compton (IC) scattering, thereby continuing the cascade process. This is often used to set limits on intergalactic magnetic fields (IGMFs). However, the picture may change if plasma instabilities, arising due to the interaction of the pairs with the intergalactic medium (IGM), cool down the electrons/positrons faster than inverse Compton scattering. As a consequence, the kinetic energy lost by the pairs to the IGM could cause a hardening in the observed gamma-ray spectrum at energies below ∼100 GeV. Here we study several types and models of instabilities and assess their impact for interpreting observations of distant blazars. Our results suggest that plasma instabilities can describe the spectra of some blazars and mimic the effects of IGMFs, depending on parameters such as the intrinsic spectrum of the object, the density and temperature of the IGM, and the luminosity of the beam. On the other hand, we find that for our fiducial set of parameters plasma instabilities do not have a major impact on the spectra of some of the blazars studied. Therefore, they may be used for constraining IGMFs.

The Gamma-ray Window to Intergalactic Magnetism

“…For the extreme blazars with hard spectra (see discussion in Section 3.1), in particular, this will ultimately reduce the uncertainties when constraining IGMFs. Interestingly, observations around GeV energies may also probe spectral features expected from some plasma instability models (e.g., [302]; see also Section 3.4). Comparison of the point-source sensitivity for various gamma-ray observatories.…”

“…Plasma instabilities are often neglected in simulations, or treated within a dedicated MHD computational framework. In [302], grplinst was presented. It is a module for the CRPropa code that implements plasma effects on the electrons as an additional energy-loss term of the form…”

“…Therefore, Equation ( 27) is overestimated, since in reality T ≤ τ. More recently, these same parametrizations of grplinst [302] were implemented in Elmag 3.02.…”

The intrinsic gamma-ray spectrum of TXS 0506+056: intergalactic propagation effects

“…Consequently, IGMFs induce a flux suppression of secondary photons; but they are not accounted for in SED fits. It should be noted here that there is another explanation for this suppression: Electrons/positrons present in the cascade might interact with the intergalactic medium and generate plasma instabilities, leading to energy losses and, consequently, to a lower flux of HE photons arriving at the observer (Broderick et al 2012;Schlickeiser et al 2012;Miniati & Elyiv 2013;Schlickeiser et al 2013;Sironi & Giannios 2014;Broderick et al 2018;Vafin et al 2018;Yan et al 2019;Alves Batista et al 2019b) .…”

Optical spectroscopy of blazars for the Cherenkov Telescope Array

“…The propagation of VHE gamma-ray radiation can be used to investigate topics related to cosmology or fundamental physics: the properties of the intergalactic magnetic field (IGMF) (see e.g. Ackermann et al 2018;Aharonian et al 1994;Alves Batista et al 2019), the possible existence of axion-like particles (see e.g. Mirizzi et al 2007;de Angelis et al 2011;Abramowski et al 2013), an independent measurement of the Hubble constant H0 (Salamon et al 1994), or the search for Lorentz Invariance Violation (Kifune 1999).…”