Simulations of astrophysically relevant pair beam instabilities in a laboratory context

Abstract

The interaction of TeV blazars emitted gamma-rays with the extragalactic background photons gives rise to a relativistic beam of electron–positron ($e^{-}{e}^{+}$) pairs propagating through the intergalactic medium, producing a cascade through up-scattering low-energy photons. Plasma instability is considered one of the underlying energy-loss processes of the beams. We employ particle-in-cell (PIC) simulations to study the plasma instabilities of relativistic pair beams propagating in a denser background plasma, using the parameters designed to replicate astrophysical jets under laboratory conditions. In an astrophysical scenario with a broad, dilute beam, electromagnetic instability is suppressed because the beam exhibits momentum anisotropy with a large longitudinal momentum spread compared to its transverse momentum. We find the range of density contrast at which electrostatic modes are dominating over electromagnetic modes with an anisotropic beam in laboratory scales, consistent with the physically relevant conditions for Blazar-induced beams. We have used a broad Cauchy distribution for the beam particles, which is more realistic in representing the non-Maxwellian nature of pair beams, improving upon previous studies. We investigate the interplay between the instability-generated magnetic field and the momentum anisotropy of the beam. We extrapolate the beam energy loss and the angular broadening due to non-linear feedback of instability. We find that the astrophysical beams have lost approximately 4% of their total energy due to instability. Nevertheless, the instability generates a negligible angular broadening for Blazar-induced beams.