The Newton polygons of overconvergent F-crystals

R. Crew conjectured that every overconvergent F-isocrystal over k((t)) (k a field of positive characteristic) is quasi-unipotent (equivalently, potentially semistable), and so has ``generic'' and ``special'' Newton polygons. It is easy to construct a Newton polygon for an arbitrary overconvergent F-crystal that coincides with the generic Newton polygon for potentially semistable crystals. We give an analogous construction for the special Newton polygon, by showing that the F-crystal can be trivialized over a large auxiliary ring. In a subsequent preprint, we use this construction to prove the aforementioned conjecture.