The Generic Isogeny Decomposition of the Prym Variety of a Cyclic Branched Covering

Abstract

Let $f\colon S'\longrightarrow S$ be a cyclic branched covering of smooth projective surfaces over $\mathbb{C}$ whose branch locus $\Delta\subset S$ is a smooth ample divisor. Pick a very ample complete linear system $|\mathcal{H}|$ on $S$, such that the polarized surface $(S, |\mathcal{H}|)$ is not a scroll nor has rational hyperplane sections. For the general member $[C]\in|\mathcal{H}|$ consider the $\mu_{n}$-equivariant isogeny decomposition of the Prym variety $Prym(C'/C)$ of the induced covering $f\colon C'= f^{-1}(C)\longrightarrow C$:\[Prym(C'/C)\sim\prod_{d|n,\ d\neq1}\mathcal{P}_{d}(C'/C).\] We show that for the very general member $[C]\in|\mathcal{H}|$ the isogeny component $\mathcal{P}_{d}(C'/C)$ is $\mu_{d}$-simple with $End_{\mu_{d}}(\mathcal{P}_{d}(C'/C))\cong\mathbb{Z}[\zeta_{d}]$. In addition, for the non-ample case we reformulate the result by considering the identity component of the kernel of the map $\mathcal{P}_{d}(C'/C)\subset Jac(C')\longrightarrow Alb(S')$.