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Abstract

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. In this work, we study the role played by the
large-scale anisotropy of the transverse magnetic field profile on the shock
dynamics. We performed large two-dimensional particle-in-cell simulations for a
wide range of upstream plasma magnetizations. 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. 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.