Speaker
Description
TeV gamma rays from blazars interact with the extragalactic background light and produce ultrarelativistic, strongly forward-directed $e^+e^-$ pairs. The (in)stability of these pair beams is important for intergalactic magnetic field (IGMF) constraints, the distribution of secondary gamma rays, and the thermal evolution of the intergalactic medium (IGM). Pair beams drive a linear instability of plasma oscillations at wave numbers $k\approx \ell_{\rm s}^{-1}\sec\theta$ ($\theta$ being the angle between the beam and wave vector, and $\ell_{\rm s}$ being the skin depth). We introduce another ingredient in the physics of pair beams: the cosmic ray electrons in the IGM. The $\sim$MeV gamma ray background can Compton scatter off of thermal electrons in the IGM, thus acting as a “guaranteed” source of cosmic ray electrons even in regions of the IGM that have not yet been shocked. We estimate that these electron cosmic rays contribute $\sim0.3\%$ of the IGM pressure at $z=2$, rising to $\sim2\%$ today. We find that these electrons cause linear Landau damping of plasma oscillations, with damping times of order $\sim 10^6$ s at wave numbers of $(1.2-5)\ell_{\rm s}^{-1}$ at cosmic mean density at $z=2$. This is orders of magnitude faster than the pair beam-driven instability can develop; it thus turns off the pair beam instability except for modes with very small $\theta$ (wave vector along the beam direction, where linear Landau damping is kinematically suppressed). We discuss ongoing work on the fate of this narrow range of modes that remain linearly unstable.
