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Abstract

We investigate the effects on electron acceleration of an electromagnetically modified termination shock (TS) in pulsar wind nebulae (PWNe). The physics of the magnetized pulsar wind simulated in a two-fluid code with a magnetic shear wave which converts in superluminal waves when the plasma density is below a certain threshold. This condition can be met for example in binary systems hosting a PWN when the pulsar is far from periastron. The modification of the TS structure leads to energization of particles incoming with the wind, and a sizeable fraction of these arc reflected and therefore made available for further acceleration. Using a Monte Carlo code, we show that this subsequent acceleration process is remarkably similar to first-order Fermi acceleration at parallel, relativistic shocks. Applying this scenario to the bright GeV flare produced by PSR B1259-63 we find a value of the pair multiplicity in the wind in good agreement with previous estimates for this source.