Abstract
Context. Nearby pulsar wind nebulae exhibit complex morphological features: jets, torus, arcs, and knots. These structures are well captured and understood in the scope of global magnetohydrodynamic models. However, the origin of knots in the inner radius of the Crab Nebula remains elusive. Aims. In this work, we investigate the dynamics of the shock front and downstream flow with a special emphasis on the reconnecting equatorial current sheet. We examine whether giant plasmoids produced in the reconnection process could be good candidates for the knots. Methods. To this end, we perform large semi-global three-dimensional particle-in-cell simulations in a spherical geometry. The hierarchical merging plasmoid model is used to extrapolate numerical results to pulsar wind nebula scales. Results. The shocked material collapses into the midplane, forming and feeding a large-scale, but thin, ring-like current layer. The sheet breaks up into a dynamical chain of merging plasmoids, reminiscent of three-dimensional reconnection. Plasmoids grow to a macroscopic size. The final number of plasmoids predicted is solely governed by the inverse of the dimensionless reconnection rate. Conclusions. The formation of giant plasmoids is a robust feature of pulsar wind termination shocks. They provide a natural explanation for the inner-ring knots in the Crab Nebula, provided that the nebula is highly magnetized.
Highlights
High-resolution X-ray images of nearby pulsar wind nebulae, as best illustrated by the iconic Chandra images of the Crab Nebula (Weisskopf et al 2000; Hester et al 2002; Mori et al 2004) and of the Vela pulsar wind nebula (Helfand et al 2001; Pavlov et al 2001a; Durant et al 2013), have uncovered complex morphological features
In this work we have shown that a large-scale ring-like current sheet forms and reconnects in the equatorial plane downstream of pulsar wind termination shocks
Assuming that reconnection is unperturbed for at most a light-crossing time of the shock radius – after which Kelvin-Helmholtz and kink instabilities will mix and disrupt the layer, leading to a turbulent state – the final number of giant plasmoids is solely determined by the inverse of the dimensionless reconnection rate, Nifs = πβ−re1c
Summary
High-resolution X-ray images of nearby pulsar wind nebulae, as best illustrated by the iconic Chandra images of the Crab Nebula (Weisskopf et al 2000; Hester et al 2002; Mori et al 2004) and of the Vela pulsar wind nebula (Helfand et al 2001; Pavlov et al 2001a; Durant et al 2013), have uncovered complex morphological features. Two-dimensional (2D) axisymmetric magnetohydrodynamic (MHD) simulations have successfully reproduced the observed jet-torus structure, which was identified as the plasma downstream of the pulsar wind termination shock (Komissarov & Lyubarsky 2003, 2004; Del Zanna et al 2004) In this framework, the moving wisps result from the feedback of the highly dynamical downstream flow on the shock front location (Camus et al 2009), while the inner knot originates from the Doppler-boosted emission of the termination shock pointing toward the observer The morphological features that are not captured in the scope of the 3D MHD model are the brightness of the inner ring and its knots (Porth et al 2014) This region is often regarded as the location of the termination shock itself in the equatorial plane (Weisskopf et al 2000). We provide a detailed analysis of their evolution and propose a toy model that can be used to apply and extrapolate our results to any pulsar wind nebula
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