Equivariant concordance of periodic 2-knots in $S^4$

We show that the smooth equivariant concordance group of 2-knots in $S^4$ invariant under a linear $\mathbb{Z}\mathrm{/}d\mathbb{Z}$ action is isomorphic to $\mathbb{Z}\mathrm{/}2\mathbb{Z}$ for all $d\ge 2$. This is in contrast to the non-equivariant case, in which all 2-knots are slice. We construct a new invariant for these 2-knots, which we call periodic, and show that it fully classifies them up to equivariant concordance. The invariant depends on a variation of the Arf invariant for null-homologous classical knots in arbitrary 3-manifolds with respect to a chosen spin structure. Our proof also shows an identical classification for certain annuli in $S^1\times B^3$ up to concordance rel. boundary.