Particle acceleration at ultrarelativistic, perpendicular shock fronts

Using an eigenfunction expansion to solve the transport equation,complemented by Monte-Carlo simulations, we show that ultrarelativistic shockscan be effective particle accelerators even when they fail to produce largeamplitude turbulence in the downstream plasma. This finding contradicts thewidely held belief that a uniform downstream magnetic field perpendicular tothe shock normal inhibits acceleration by the first order Fermi process. In theultrarelativistic limit, we find a stationary power-law particle spectrum ofindex s=4.17 for these shocks, close to that predicted for a strictly parallelshock.