The Whittaker functional is a shifted microstalk

Abstract

For a smooth projective curve $X$ and reductive group $G$, the Whittaker functional on nilpotent sheaves on ${\text{Bun}}_G(X)$ is expected to correspond to global sections of coherent sheaves on the spectral side of Betti geometric Langlands. We prove that the Whittaker functional calculates the (shifted) microstalk of nilpotent sheaves at the point in the Hitchin moduli where the Kostant section intersects the global nilpotent cone. In particular, the (shifted) Whittaker functional is exact for the perverse $t$-structure and commutes with Verdier duality. Our proof is topological and depends on the intrinsic local hyperbolic symmetry of ${\text{Bun}}_G(X)$. It is an application of a general result relating vanishing cycles to the composition of restriction to an attracting locus followed by vanishing cycles.

This is an update to the published version with some errors corrected. We are grateful to Swapnil Garg for his careful reading and pointing out of errors