Constraints on particle acceleration sites in the Crab nebula from relativistic magnetohydrodynamic simulations

Abstract

The Crab Nebula is one of the most efficient accelerators in the Galaxy and\nthe only galactic source showing direct evidence of PeV particles. In spite of\nthis, the physical process behind such effective acceleration is still a deep\nmystery. While particle acceleration, at least at the highest energies, is\ncommonly thought to occur at the pulsar wind termination shock, the properties\nof the upstream flow are thought to be non-uniform along the shock surface, and\nimportant constraints on the mechanism at work come from exact knowledge of\nwhere along this surface particles are being accelerated. Here we use\naxisymmetric relativistic MHD simulations to obtain constraints on the\nacceleration site(s) of particles of different energies in the Crab Nebula.\nVarious scenarios are considered for the injection of particles responsible for\nsynchrotron radiation in the different frequency bands, radio, optical and\nX-rays. The resulting emission properties are compared with available data on\nthe multi wavelength time variability of the inner nebula. Our main result is\nthat the X-ray emitting particles are accelerated in the equatorial region of\nthe pulsar wind. Possible implications on the nature of the acceleration\nmechanism are discussed.\n