On page 55 of David Tong's String Theory lecture notes, during the discussion of the first excited states of open strings, two classes of states are identified, both of which are massless. The first class is described as,
Oscillators longitudinal to the brane, $$\alpha_{-1}^{a}|0 ; p\rangle \quad a=1, \ldots, p-1$$ The spacetime indices a lie within the brane so this state transforms under the $SO(1, p)$ Lorentz group. It is a spin 1 particle on the brane or, in other words, it is a photon. We introduce a gauge field $A_a$ with $a=0, \ldots, p$ lying on the brane whose quanta are identified with this photon.
It is not obvious to me that this state is a spin 1 particle. Why is it spin 1? Is that clear from the symmetry of the $SO(1,p)$ Lorentz group? Is this related to the Wigner classification? I think my not recognizing this might point to a gap in my QFT knowledge.