Compton Scattering of Electrons in the Intergalactic Medium

Abstract

This paper investigates the distribution and implications of cosmic-ray electrons within the intergalactic medium (IGM). Utilizing a synthesis model of the extragalactic background, we evolve the spectrum of Compton-included cosmic rays. The energy density distribution of cosmic-ray electrons peaks at redshift z ≈ 2, and peaks in the ∼MeV range. The fractional contribution of cosmic-ray pressure to the general IGM pressure progressively increases toward lower redshift. At mean density, the ratio of cosmic-ray electron to thermal pressure in the IGM ${P}_{\mathrm{CRe}}/{P}_{\mathrm{th}}$ is 0.3% at z = 2, rising to 1.0% at z = 1, and 1.8% at z = 0.1 (considering only the cosmic rays produced locally by Compton scattering). We compute the linear Landau damping rate of plasma oscillations in the IGM caused by the ∼MeV cosmic-ray electrons, and find it to be of order ∼10 ^−6 s ^−1 for wavenumbers 1.2 ≲ ck / ω _p ≲ 5 at z = 2 and mean density (where ω _p is the plasma frequency). This strongly affects the fate of TeV e ^+ e ^− pair beams produced by blazars, which are potentially unstable to oblique instabilities involving plasma oscillations with wavenumber ${ck}/{\omega }_{{\rm{p}}}\approx \sec \theta $ ( θ being the angle between the beam and wavevector). Linear Landau damping is at least thousands of times faster than either pair beam instability growth or collisional effects; it thus turns off the pair beam instability except for modes with very small θ ( ck / ω _p → 1, where linear Landau damping is kinematically suppressed). This leaves open the question of whether the pair beam instability is turned off entirely, or can still proceed via the small- θ modes.

Keywords

• Cosmic rays
• Intergalactic gas
• Cosmic magnetic fields theory