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Abstract: We present evidence that relativistic shocks propagating in unmagnetizedplasmas can self-consistently accelerate particles. We use long-termtwo-dimensional particle-in-cell simulations to study the well-developed shockstructure in unmagnetized pair plasma. The particle spectrum downstream of sucha shock consists of two components: a relativistic Maxwellian, withcharacteristic temperature set by the upstream kinetic energy of the flow, anda high-energy tail, extending to energies >100 times that of the thermal peak.This tail is best fitted as a power law in energy with index -2.4+-0.1,modified by an exponential cutoff. The cutoff moves to higher energies withtime of the simulation, leaving a larger power law range. The number ofparticles in the tail is ~1% of the downstream population, and they carry ~10%of the kinetic energy in the downstream. Upon investigation of the trajectoriesof particles in the tail, we find that the energy gains occur as particlesbounce between the upstream and downstream regions in the magnetic fieldsgenerated by the Weibel instability. We compare this mechanism to the firstorder Fermi acceleration, and set a lower limit on the efficiency of shockacceleration process.



Autor: Anatoly Spitkovsky Princeton University

Fuente: https://arxiv.org/







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