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Description
Relativistic shocks are not efficient cosmic-ray accelerators because after the first shock crossing cycle the particles do not have time to isotropise in the shock upstream region. However, in the first cycle the particles get a boost $\propto \Gamma^2$, where $\Gamma$ is the Lorentz factor of the shock. We construct a model in which particles can achieve ultra-high energies by passing through multiple relativistic shocks in the jet of the nearest radiogalaxy Centaurus A.
Centaurus A is the result of a collision of two normal galaxies resulting in a fantastic jumble of star clusters. When young stars and clusters encounter the jets, the jet-ram pressure confines the stellar wind, creating a relativistic bowshock in which the mechanism described above can operate. The maximum energy depends on the number of bowshocks a single particle encounters. We show that for conservative stellar distributions and a jet Lorentz factor of about 10, we can explain particle acceleration up to ultra-high energies. This scenario has the advantage that the mass composition of UHECRs is similar to that of massive stellar winds.
