Relativistic Shear Flow Between Electron-Ion and Electron-Positron Plasmas and Astrophysical Applications

TL;DR

This paper uses Particle-in-Cell simulations to study relativistic shear layers in plasmas, revealing how electron-positron and electron-ion interactions produce anisotropic lepton acceleration and angle-dependent radiation spectra relevant to astrophysical jets.

Contribution

It provides new insights into particle acceleration and radiation beaming in relativistic shear layers, contrasting electron-positron and electron-ion plasma behaviors.

Findings

Lepton acceleration is highly anisotropic at shear interfaces.

High-energy leptons produce narrow beaming patterns of order 1/Γ.

Shear-layer acceleration results in angle-dependent, harder spectra in the forward direction.

Abstract

We present Particle-in-Cell simulation results of relativistic shear boundary layers between electron-ion and electron-positron plasmas and discuss their potential applications to astrophysics. Specifically, we find in the case of a fast electron-positron spine surrounded by a slow-moving or stationary electron-ion sheath, lepton acceleration proceeds in a highly anisotropic manner due to electromagnetic fields created at the shear interface. While the highest-energy leptons still produce a beaming pattern (as seen in the quasi-stationary frame of the sheath) of order 1/{\Gamma}, where {\Gamma} is the bulk Lorentz factor of the spine, for lower-energy particles, the beaming is much less pronounced. This is in stark contrast to the case of pure electron-ion shear layers, in which anisotropic particle acceleration leads to significantly narrower beaming patterns than 1/{\Gamma} for the highest-energy particles. In either case, shear-layer acceleration is expected to produce strongly angle-dependent lepton (and hence, emanating radiation) spectra, with a significantly harder spectrum in the forward direction than viewed from larger off-axis angles, much beyond the regular Doppler boosting effect from a co-moving isotropic lepton distribution. This may solve the problem of the need for high (and apparently arbitrarily chosen) minimum Lorentz factors of radiating electrons, often plaguing current blazar and GRB jet modeling efforts.