A note about the isotropy groups of $2$-plane bundles over closed surfaces

Let $\xi$ be a $2$-plane bundle over a closed surface $S$. The line bundles $\lambda$ over $S$ such that $\xi\otimes\lambda\cong\xi$ form a group $\mathcal{J}(\xi)$ (the isotropy group of $\xi$); the scope of this paper is to describe $\mathcal{J}(\xi)$.