Energy transport in a rotation-modulated pulsar wind

Abstract

A rotation-powered pulsar generates large-amplitude, oscillating electromagnetic fields in its relativistic wind. The importance of these fields in transporting energy within the wind and determining the structure of the wind is investigated here. It is shown that the theory of ideal magnetohydrodynamics (MHD) underpinning standard, steady-state wind models does not apply throughout the wind and must be replaced by an exact one-fluid theory that includes a generalized, relativistic Ohm's law. It is also shown, by constructing the wind artificially from the pulsar's vacuum fields by adding test particles, that the wind displacement current asymptotically dominates the conduction current; that Poynting flux is asymptotically transported by large-amplitude, transverse electromagnetic waves which strongly accelerate particles (i.e., an outer radiation zone); that the inner boundary of the radiation zone lies well inside the wind termination shock for the Crab pulsar; and that at lesser distances, where transverse waves are catastrophically damped, the wind displacement current is associated with longitudinal oscillating fields. A self-consistent fluid model of the radiation zone predicts that the kinetic energy flux dominates the Poynting flux if the Lorentz factor of the flow is large and there is no local heating. Thus the existence of a radiation zone partially explains a feature of the Crab pulsar wind that has hitherto seemed paradoxical in the context of standard wind models - namely, that the flow is Poynting-flux-dominated at the light cylinder by virtue of its generation in pair cascades, yet needs to be kinetic-flux-dominated at the termination shock if pressure confinement of the Crab nebula by its supernova remnant is to be self-consistently achieved. However, a complete resolution of the paradox must await the identification of a plausible flux conversion process, an issue that this paper does not address.