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

摘要

The interaction of TeV gamma rays from distant blazars with the extragalacticbackground light produces relativistic electron-positron pair beams by thephoton-photon annihilation process. Using the linear instability analysis inthe kinetic limit, which properly accounts for the longitudinal and the smallbut finite perpendicular momentum spread in the pair momentum distributionfunction, the growth rate of parallel propagating electrostatic oscillations inthe intergalactic medium is calculated. Contrary to the claims of Miniati andElyiv (2013) we find that neither the longitudinal nor the perpendicular spreadin the relativistic pair distribution function do significantly affect theelectrostatic growth rates. The maximum kinetic growth rate for noperpendicular spread is even about an order of magnitude greater than thecorresponding reactive maximum growth rate. The reduction factors to themaximum growth rate due to the finite perpendicular spread in the pairdistribution function are tiny, and always less than $10^{-4}$. We confirm theearlier conclusions by Broderick et al. (2012) and us, that the created pairbeam distribution function is quickly unstable in the unmagnetizedintergalactic medium. Therefore, there is no need to require the existence ofsmall intergalactic magnetic fields to scatter the produced pairs, so that theexplanation (made by several authors) of the FERMI non-detection of the inverseCompton scattered GeV gamma rays by a finite deflecting intergalactic magneticfield is not necessary. In particular, the various derived lower bounds for theintergalactic magnetic fields are invalid due to the pair beam instabilityargument.