A new type of self-similarity is found in the problem of a plane-parallel, ultra-relativistic blastwave, propagating in a power-law density profile of the form ρ z k. Self-similar solutions of the first kind can be found for k < 7∕4 using dimensional considerations. For steeper density gradients with k > 2, second type solutions are obtained by eliminating a singularity from the equations. However, for intermediate power-law indices 7 / 4 < k < 2, the flow does not obey any of the known types of self-similarity. Instead, the solutions belong to a new class in which the self-similar dynamics are dictated by the non-self-similar part of the flow. We obtain an exact solution to the ultra-relativistic fluid equations and find that the non-self-similar flow is described by a relativistic expansion into vacuum, composed of (1) an accelerating piston that contains most of the energy and (2) a leading edge of a fast material that coincides with the interiors of the blastwave and terminates at the shock. The dynamics of the piston itself are self-similar and universal and do not depend on the external medium. The exact solution of the non-self-similar flow is used to solve for the shock in the new class of solutions.

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