Lepton Acceleration in Pulsar Wind Nebulae

TL;DR

This paper investigates how relativistic shocks in pulsar wind nebulae accelerate leptons, using Monte Carlo simulations to match observations and explore the influence of turbulence and shock parameters.

Contribution

It provides a detailed analysis of relativistic shock acceleration in PWNe, highlighting the role of turbulence and shock conditions in lepton energization, supported by simulation and observational constraints.

Findings

Shock acceleration can explain observed lepton energies in PWNe.

Turbulence levels influence the spectral indices of accelerated particles.

The termination shock is a plausible primary source of energetic leptons.

Abstract

Pulsar Wind Nebulae (PWNe) act as calorimeters for the relativistic pair winds emanating from within the pulsar light cylinder. Their radiative dissipation in various wavebands is significantly different from that of their pulsar central engines: the broadband spectra of PWNe possess characteristics distinct from those of pulsars, thereby demanding a site of lepton acceleration remote from the pulsar magnetosphere. A principal candidate for this locale is the pulsar wind termination shock, a putatively highly-oblique, ultra-relativistic MHD discontinuity. This paper summarizes key characteristics of relativistic shock acceleration germane to PWNe, using predominantly Monte Carlo simulation techniques that compare well with semi-analytic solutions of the diffusion-convection equation. The array of potential spectral indices for the pair distribution function is explored, defining how these depend critically on the parameters of the turbulent plasma in the shock environs. Injection efficiencies into the acceleration process are also addressed. Informative constraints on the frequency of particle scattering and the level of field turbulence are identified using the multiwavelength observations of selected PWNe. These suggest that the termination shock can be comfortably invoked as a principal injector of energetic leptons into PWNe without resorting to unrealistic properties for the shock layer turbulence or MHD structure.