High sigma model of pulsar wind nebulae

TL;DR

This paper proposes a high sigma model for pulsar wind nebulae where magnetic flux is dissipated through reconnection in specific regions, explaining observed structures without requiring low magnetization shocks.

Contribution

It introduces a novel high magnetization wind model with localized reconnection zones, challenging the traditional sigma problem and aligning with recent observational data.

Findings

Model reproduces Crab nebula morphology qualitatively

Reconnection regions correspond to observed tori and jets

Dissipation occurs in a relativistically moving wind

Abstract

Pulsars and central engines of long gamma ray burst -- collapsars -- may produce highly magnetized (Poynting flux dominated) outflows expanding in a dense surrounding (interstellar medium or stellar material). For certain injection conditions, the magnetic flux of the wind cannot be accommodated within the cavity. In this case, ideal (non-dissipative) MHD models, similar to the Kennel and Coroniti (1984) model of the Crab nebular, break down (the so-called sigma problem). This is typically taken to imply that the wind should become particle-dominated on scales much smaller than the size of the cavity. The wind is then slowed down by a fluid-type (low magnetization) reverse shock. Recent Fermi results, indicating that synchrotron spectrum of the Crab nebula extends well beyond the upper limit of the most efficient radiation reaction-limited acceleration, contradict the presence of a low sigma reverse shock. We propose an alternative possibility, that the excessive magnetic flux is destroyed in a reconnection-like process in two regions: near the rotational axis and near the equator. We construct an example of such highly magnetized wind having two distinct reconnection regions and suggest that these reconnection cites are observed as tori and jets in pulsar wind nebulae. The model reproduces, qualitatively, the observed morphology of the Crab nebula. In parts of the nebular the dissipation occurs in a relativistically moving wind, alleviating the requirements on the acceleration rate.