Colloquium by Shuta Tanaka: The Role of Turbulence in Solving Classical Problems of Pulsar Wind Nebulae

Abstract:
Non-thermal electromagnetic radiation extending up to PeV gamma rays from pulsar wind nebulae (PWNe) is powered by relativistic electron–positron plasma supplied by their central pulsars. One of the classical problems of PWNe, which we refer to here as the "kappa problem", concerns the origin of the radio-emitting particles and their large number, which greatly exceeds the number expected from pair cascades in the pulsar magnetosphere. A stochastic acceleration model has been proposed in which low-energy electrons responsible for the radio emission are injected from the supernova ejecta through photoionization and subsequently accelerated by turbulence within the nebula. Another classical problem is the well-known "sigma problem", namely, how to reconcile the high magnetization expected in the pulsar wind with the relatively slow expansion velocities observed for PWNe. The development of magnetic turbulence inside a PWN can reduce the effective magnetic tension force, allowing the nebula to expand slowly enough to reproduce the observations without requiring a low magnetization of the relativistic electron–positron plasma. In this talk, I will present our picture of the role of turbulence in PWNe and discuss how it may provide a unified framework for addressing these two classical problems. I will also present results from applying this picture to the Crab Nebula.