Plasma effects on fast pair beams II. Reactive versus kinetic instability of parallel electrostatic wave

TL;DR

This paper analyzes the electrostatic instability of relativistic pair beams in the intergalactic medium, demonstrating that kinetic effects do not significantly alter growth rates and confirming the beam's rapid instability without requiring magnetic field scattering.

Contribution

It provides a kinetic analysis of pair beam instabilities, showing that previous reactive estimates are valid and that magnetic fields are unnecessary to explain observational data.

Findings

Kinetic growth rates are higher than reactive ones.

Finite perpendicular momentum spread has negligible effect on growth rates.

Pair beams are quickly unstable in the intergalactic medium.

Abstract

The interaction of TeV gamma rays from distant blazars with the extragalactic background light produces relativistic electron-positron pair beams by the photon-photon annihilation process. Using the linear instability analysis in the kinetic limit, which properly accounts for the longitudinal and the small but finite perpendicular momentum spread in the pair momentum distribution function, the growth rate of parallel propagating electrostatic oscillations in the intergalactic medium is calculated. Contrary to the claims of Miniati and Elyiv (2013) we find that neither the longitudinal nor the perpendicular spread in the relativistic pair distribution function do significantly affect the electrostatic growth rates. The maximum kinetic growth rate for no perpendicular spread is even about an order of magnitude greater than the corresponding reactive maximum growth rate. The reduction factors to the maximum growth rate due to the finite perpendicular spread in the pair distribution function are tiny, and always less than $10^{-4}$. We confirm the earlier conclusions by Broderick et al. (2012) and us, that the created pair beam distribution function is quickly unstable in the unmagnetized intergalactic medium. Therefore, there is no need to require the existence of small intergalactic magnetic fields to scatter the produced pairs, so that the explanation (made by several authors) of the FERMI non-detection of the inverse Compton scattered GeV gamma rays by a finite deflecting intergalactic magnetic field is not necessary. In particular, the various derived lower bounds for the intergalactic magnetic fields are invalid due to the pair beam instability argument.