MeV cosmic-ray electrons modify the TeV pair-beam plasma instability

TL;DR

This paper investigates how MeV cosmic-ray electrons suppress certain plasma instabilities in intergalactic pair beams, revealing that Landau damping enhances the energy loss of these beams and affects gamma-ray observations.

Contribution

It demonstrates that linear Landau damping suppresses oblique electrostatic modes, increasing the efficiency of plasma instabilities in pair beams from blazars.

Findings

LLD suppresses oblique electrostatic modes

Quasi-parallel modes grow larger due to LLD

Instability energy-loss efficiency increases by over tenfold

Abstract

Relativistic pair beams created in the intergalactic medium (IGM) by TeV gamma rays from blazars are expected to produce a detectable GeV-scale electromagnetic cascade, but the cascade component is absent in the spectra of many hard-spectrum TeV-emitting blazars. One common explanation is that weak intergalactic magnetic fields deflect the electron-positron pairs away from our line of sight. An alternative possibility is that electrostatic beam-plasma instabilities drain the energy of these pairs before a cascade can develop. Recent studies have shown that beam scattering by oblique electrostatic modes leads to minimal energy loss. But these modes might be suppressed by linear Landau damping (LLD) due to MeV-scale cosmic-ray electrons in the IGM. In this work, we explore the impact of LLD on the energy-loss efficiency of plasma instabilities in pair beams associated with 1ES 0229+200. We find that LLD effectively suppresses oblique electrostatic modes, while quasi-parallel ones grow to larger amplitudes. In this way, LLD enhances the energy-loss efficiency of the instability by more than an order of magnitude.