A Hodge-Type Filtration on Rigid Cohomology

Given a scheme X over a complete discrete valuation ring ${\mathcal O}_K$ of mixed characteristic,Gros,in his study of syntomic cohomology in the smooth and proper case, introduced a filtration on the rigid cohomology of the special fiber $X_k$.Gros claimed that this construction was independent of the choice of immersions involved, citing an unpublished paper of Berthelot. However no proofs of this have appeared: here we prove it.