The Nahm transform of multi-fractional instantons

We embed the multi-fractional instantons of $SU(N)$ gauge theories on $\mathbb T^4$ with 't Hooft twisted boundary conditions into $U(N)$ bundles and use the Nahm transform to study the corresponding configurations on the dual $\widehat{\mathbb T}^4$. We first show that $SU(N)$ fractional instantons of topological charge $Q={r \over N}$, $r \in \{1, 2,...,N-1\}$, are mapped to fractional instantons of $SU(\widehat N)$ of charge $\widehat Q = {r \over \widehat N}$, where $\widehat N = N q_1 q_3 - r q_3 + q_1$ and $q_{1,3}$ are integer-quantized $U(1)$ fluxes. We then explicitly construct the Nahm transform of constant field strength fractional instantons of $SU(N)$ and find the $SU(\widehat N)$ configurations they map to. Both the $\mathbb T^4$ instantons and their $\widehat {\mathbb T}^4$ images are self-dual for appropriately tuned torus periods. The Nahm duality can be extended to tori with detuned periods, with detuning parameter $\Delta$, mapping solutions with $\Delta >0$ on $\mathbb T^4$ to ones with $\widehat\Delta <0$ on $\widehat{\mathbb T}^4$. We also recall that fractional instantons appear in string theory precisely via the $U(N)$ embedding, suggesting that studying the end point of tachyon condensation for $\Delta \ne 0$ is needed -- and is perhaps feasible in a small-$\Delta$ expansion, as in field theory studies -- in order to understand the appearance and role of fractional instantons in $D$-brane constructions.