Abstract
Context. Pulsar wind nebulae are efficient particle accelerators, and yet the processes at work remain elusive. Self-generated, microturbulence is too weak in relativistic magnetized shocks to accelerate particles over a wide energy range, suggesting that the global dynamics of the nebula may be involved in the acceleration process instead. Aims. In this work, we study the role played by the large-scale anisotropy of the transverse magnetic field profile on the shock dynamics. Methods. We performed large two-dimensional particle-in-cell simulations for a wide range of upstream plasma magnetizations, from weakly magnetized to strongly magnetized pulsar winds. Results. The magnetic field anisotropy leads to a dramatically different structure of the shock front and downstream flow. A large-scale velocity shear and current sheets form in the equatorial regions and at the poles, where they drive strong plasma turbulence via Kelvin-Helmholtz vortices and kinks. The mixing of current sheets in the downstream flow leads to efficient nonthermal particle acceleration. The power-law spectrum hardens with increasing magnetization, akin to those found in relativistic reconnection and kinetic turbulence studies. The high end of the spectrum is composed of particles surfing on the wake produced by elongated spearhead-shaped cavities forming at the shock front and piercing through the upstream flow. These particles are efficiently accelerated via the shear-flow acceleration mechanism near the Bohm limit. Conclusions. Magnetized relativistic shocks are very efficient particle accelerators. Capturing the global dynamics of the downstream flow is crucial to understanding them, and therefore local plane parallel studies may not be appropriate for pulsar wind nebulae and possibly other astrophysical relativistic magnetized shocks. A natural outcome of such shocks is a variable and Doppler-boosted synchrotron emission at the high end of the spectrum originating from the shock-front cavities, reminiscent of the mysterious Crab Nebula gamma-ray flares.
Highlights
Pulsar wind nebulae are archetypal cosmic particle accelerators
Self-generated, microturbulence is too weak in relativistic magnetized shocks to accelerate particles over a wide energy range, suggesting that the global dynamics of the nebula may be involved in the acceleration process instead
The mixing of current sheets in the downstream flow leads to efficient nonthermal particle acceleration
Summary
Pulsar wind nebulae are archetypal cosmic particle accelerators. The most studied amongst them, the Crab Nebula, presents one of the best known examples of a purely nonthermal emission spectrum extending over 20 orders of magnitude in frequency range, from 100 MHz radio waves to 100 TeV gamma rays (Meyer et al 2010). The idea of driven magnetic reconnection within the largescale pulsar wind current sheet at the shock front has been considered to circumvent the above difficulties (Lyubarsky 2003; Pétri & Lyubarsky 2007), but this scenario requires an unusually high pair plasma supply (Sironi & Spitkovsky 2011) and assumes that negligible dissipation took place in the current sheet before the shock, which may not happen (Coroniti 1990; Cerutti & Philippov 2017) Another particle acceleration mechanism involves electron acceleration by the absorption of ion cyclotron waves emitted at the shock front (Hoshino et al 1992; Amato & Arons 2006), but this model requires a high injection rate of ions in the wind.
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