$\eta$-invariant and Modular Forms

We show that the Atiyah-Patodi-Singer reduced $\eta$-invariant of the twisted Dirac operator on a closed $4m-1$ dimensional spin manifold, with the twisted bundle being the Witten bundle appearing in the theory of elliptic genus, is a meromorphic modular form of weight $2m$ up to an integral $q$-series. We prove this result by combining our construction of certain modular characteristic forms associated to a generalized Witten bundle on spin$^c$-manifolds with a deep topological theorem due to Hopkins.