General rotating black holes in string theory: Greybody factors and event horizons

We derive the wave equation for a minimally coupled scalar field in the background of a general rotating five-dimensional black hole. It is written in a form that involves two types of thermodynamic variables, defined at the inner and outer event horizon, respectively. We model the microscopic structure as an effective string theory, with the thermodynamic properties of the left- and right-moving excitations related to those of the horizons. Previously known solutions to the wave equation are generalized to the rotating case, and their regime of validity is sharpened. We calculate the greybody factors and interpret the resulting Hawking emission spectrum microscopically in several limits. We find a $U$-duality-invariant expression for the effective string length that does not assume a hierarchy between the charges. It accounts for the universal low-energy absorption cross section in the general nonextremal case.