Brauer groups of smooth loci in linear systems and torsors over Jacobians of plane curves

Abstract

We study Brauer groups of the smooth loci in linear systems on simply connected smooth projective varieties. Under a suitable ampleness condition, we prove that the Brauer group is at most $\mathbb{Z}/2\mathbb{Z}$. This applies when the underlying variety is the projective plane, a very general K3 surface, or a general cubic fourfold. As an application, we compute the Tate--Shafarevich group parametrizing torsors over the relative Jacobians of universal smooth plane curves. Our approach is via a study of the $2$-nodal locus in the linear system.

Result for K3 surfaces strengthened via a tacnode monodromy argument. 28 pages, comments are welcome!