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 $\mathbb{S}^1$ 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? 

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  [1]: https://i.sstatic.net/pjJ5s.png