Torons and black hole entropy

We consider a supersymmetric system of D-5-branes compactified on T-4 x S-1 With a self-dual background field strength on T4 and carrying left-moving momentum along S-1. The corresponding supergravity solution describes a five-dimensional black hole with a regular horizon. The entropy of this black hole may be explained in terms of the Landau degeneracy for open strings stretching between different branes. In the gauge theory approximation this D-5-brane system is described by a super Yang-Mills theory with a 't Hooft twist. By choosing a supersymmetric branch of the theory we obtain perfect agreement with the entropy formula. The result relies on the number of massless torons associated with the gauge field components that obey twisted boundary conditions. (C) 1998 Elsevier Science B.V.