Particle acceleration in relativistic collisionless shocks: Fermi process at last?

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We present evidence that relativistic shocks propagating in unmagnetized plasmas can self-consistently accelerate particles.

• We use long-term two-dimensional particle-in-cell simulations to study the well-developed shock structure in unmagnetized pair plasma.
• The particle spectrum downstream of such a shock consists of two components: a relativistic Maxwellian, with characteristic temperature set by the upstream kinetic energy of the flow, and a 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.