Local Tate duality for syntomic cohomology in positive characteristic

Let $k$ be a local field of characteristic $p>0$. We show that syntomic cohomology over $k$, with coefficients, and defined using Dodd’s theory of $F$-gauges, satisfies local Tate duality. We also prove that syntomic cohomology over the ring of integers of $k$, again with coefficients, is Lagrangian with respect to the duality pairing. Finally, we show that, for $p>2$, these results recover Shatz’s duality and Milne’s orthogonality results for the cohomology of finite flat group schemes of $p$-power order over $k$.