The Néron model of a higher-dimensional Lagrangian fibration

Abstract

Let $π: X \to B$ be a projective Lagrangian fibration of a smooth symplectic variety $X$ to a smooth variety $B$. Denote the complement of the discriminant locus by $B_0 = B \setminus \operatorname{Disc}(π)$, its preimage by $X_0 = π^{-1}(B_0)$, and the complement of the critical locus by $X' = X \setminus \operatorname{Sing}(π)$. Under an assumption that the morphism $X' \to B$ is surjective, we construct (1) the Néron model of the abelian fibration $π_0 : X_0 \to B_0$ and (2) the Néron model of its automorphism abelian scheme $\operatorname{Aut}^{\circ}_{π_0} \to B_0$. Contrary to the case of elliptic fibrations, $X'$ may not be the Néron model of $X_0$; this is precisely because of the existence of flops in higher-dimensional symplectic varieties. Using such techniques, we analyze when $X' \to B$ is a torsor under a smooth group scheme and also revisit some known results in the literature.