Complete Integrability of the Parahoric Hitchin System

Abstract

Abstract Let $\mathcal {G}$ be a parahoric group scheme over a complex projective curve X of genus greater than one. Let $\mathrm {Bun}_{\mathcal {G}}$ denote the moduli stack of $\mathcal {G}$-torsors on X. We prove several results concerning the Hitchin map on $T^{\ast }\!\mathrm {Bun}_{\mathcal {G}}$. We first show that the parahoric analogue of the global nilpotent cone is isotropic and use this to prove that $\mathrm {Bun}_{\mathcal {G}}$ is “very good” in the sense of Beilinson–Drinfeld. We then prove that the parahoric Hitchin map is a Poisson map whose generic fibres are abelian varieties. Together, these results imply that the parahoric Hitchin map is a completely integrable system.