Symplectic forms invariant under free circle actions on 4--manifolds

Let $M^4$ be a smooth compact manifold with a free circle action generated by a vector field $X.$ Then for any invariant symplectic form $\omega$ on $M$ the contracted form $\iota_X \omega$ is nondegenerate. Using the map $\omega --> \iota_X \omega$ and the related map to $H^1(M / S^1, R)$ we study the topology of the space $S_{inv}$ of invariant symplectic forms. In particular we give a description of $\pi_0 S_{inv}(M^4)$ in terms of the unit Thurston ball.