There is a claim in the following thesis regarding the exact sequence of Gysin. Shouldn't the spherical bundle $\mathbb{S}^1 \rightarrow X \rightarrow X/\mathbb{S}^1$ be orientable for the Gysin exact sequence?

I read in some sources that the spherical bundle $\mathbb{S}^1 \rightarrow X \rightarrow X/\mathbb{S}^1$ is orientable for free action of $\mathbb{S}^1$ on $X$, but I could not find a complete reference.

Under what conditions, $\mathbb{S}^1 \rightarrow X \rightarrow X/\mathbb{S}^1$ is orientable fiber bundle. Is it enough to be almost free?