The N\'eron model of a higher-dimensional Lagrangian fibration

Abstract

Let $\pi : X \to B$ be a projective Lagrangian fibration of a smooth symplectic variety $X$ to a smooth variety $B$. Denote the complement of the discriminant locus by $B_0 = B \setminus \operatorname{Disc}(\pi)$, its preimage by $X_0 = \pi^{-1}(B_0)$, and the complement of the critical locus by $X' = X \setminus \operatorname{Sing}(\pi)$. Under an assumption that the morphism $X' \to B$ is surjective, we construct (1) the N\'eron model of the abelian fibration $\pi_0 : X_0 \to B_0$ and (2) the N\'eron model of its automorphism abelian scheme $\operatorname{Aut}^{\circ}_{\pi_0} \to B_0$. Contrary to the case of elliptic fibrations, $X'$ may not be the N\'eron 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.