Pulsar wind nebulae (PWNe) are known to accelerate electrons to very high energies (VHE), but the acceleration mechanism and site remain uncertain. The pulsar wind termination shock (TS) is a natural candidate for the acceleration site, but the toroidal geometry of the magnetic field there should render shock acceleration inoperative. In this talk, we present a novel solution to this apparent contradiction. Integrating individual particle trajectories in a model of the magnetic field and flow pattern inspired by MHD simulations of PWNe, we find that drift motion along the shock surface keeps either electrons or positrons in a ring-shaped region of the TS, close to the equatorial plane of the pulsar, where they are accelerated to VHE by the first-order Fermi mechanism. Applying our findings to the Crab Nebula, we find that both its high-energy synchrotron emission and > TeV gamma-ray emission can be reproduced by this model. We show that the recent observations by LHAASO of the Crab Nebula up to PeV energies allow for putting new constraints on parameters of the Crab pulsar wind that are still poorly known. Finally, we present results from our heavier Particle-In-Cell simulations of pulsar wind TS. They confirm the above results, and also suggest that a second electron acceleration mechanism (namely, reconnection) operates downstream of the TS, in the equatorial current sheet. We show that the latter provides a natural explanation for the inner-ring knots in the Crab Nebula, whose origin had remained elusive so far.
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Meeting ID: 921 1801 0804