Irradiation models for ULXs and fits to HST observations of NGC 4559 X-7

We detail a model we have created to describe the optical emission from a ULX in terms of an irradiated companion star and disk. We apply this model to optical observations of ULX X-7 in NGC 4559. We revise the parameters of the companion star in this system to be older, less massive and of a later spectral type than previously reported. We find the black hole to be of a few hundred solar masses at most.