Kinetic simulations of electron-positron induced streaming instability in the context of gamma-ray halos around pulsars

TL;DR

This study uses kinetic simulations to explore electron-positron streaming instabilities that could explain extended gamma-ray halos around pulsars, revealing wave behaviors and particle dynamics relevant to astrophysical turbulence.

Contribution

First fully kinetic 1D3V PIC simulations of electron-positron streaming instability in pulsar environments, confirming analytical predictions and revealing detailed wave-particle interactions.

Findings

Wave spectrum includes Alfvénic and kinetic scales.

Left-handed waves are damped at ion-cyclotron branch.

Positrons exhibit different dynamics than electrons.

Abstract

The possibility of slow diffusion regions as the origin for extended TeV emission halos around some pulsars (such as PSR J0633+1746 and PSR B0656+14) challenges the standard scaling of the electron diffusion coefficient in the interstellar medium. Self-generated turbulence by electron-positron pairs streaming out of the pulsar wind nebula was proposed as a possible mechanism to produce the enhanced turbulence required to explain the morphology and brightness of these TeV halos. We perform fully kinetic 1D3V particle-in-cell simulations of this instability, considering the case where streaming electrons and positrons have the same density. This implies purely resonant instability as the beam does not carry any current. We compare the linear phase of the instability with analytical theory and find very reasonable agreement. The non-linear phase of the instability is also studied, which reveals that the intensity of saturated waves is consistent with a momentum exchange criterion between a decelerating beam and growing magnetic waves. With the adopted parameters, the instability-driven wavemodes cover both the Alfv\'enic (fluid) and kinetic scales. The spectrum of the produced waves is non-symmetric, with left-handed circular polarisation waves being strongly damped when entering the ion-cyclotron branch, while right-handed waves are suppressed at smaller wavelength when entering the Whistler branch. The low-wavenumber part of the spectrum remains symmetric when in the Alfv\'enic branch. As a result, positrons behave dynamically differently compared to electrons. We also observed a second harmonic plasma emission in the wave spectrum. An MHD-PIC approach is warranted to probe hotter beams and investigate the Alfv\'en branch physics. This work confirms that the self-confinement scenario develops essentially according to analytical expectations [...](abridged)