A high-sigma model of pulsar wind nebulae
Monthly Notices of the Royal Astronomical Society 405,3 (2010) 1809-1815;
Pulsars and central engines of long gamma-ray bursts - collapsars - may produce highly magnetized (Poynting-flux-dominated) outflows expanding in dense surroundings (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) magnetohydrodynamics models, similar to the Kennel & Coroniti model of the Crab nebula, 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 the 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 in nearly aligned pulsars 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 a highly magnetized wind having two distinct reconnection regions and suggest that these reconnection sites 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 nebula dissipation occurs in a relativistically moving wind, alleviating requirements on the acceleration rate.
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