Changes in the X-ray irradiation of an ultraluminous X-ray source

Optical emission observed from ultraluminous X-ray sources (ULXs) comes from the irradiated disk and the donor star. Disentangling the two components has always been an unsolved problem. We have discovered that the ULX NGC 1313 X-2 switches between two distinct X-ray spectral/luminosity states on long timescales, ~ 6--10 weeks. This makes it an ideal laboratory to study the effects of variable X-ray irradiation on the disk and donor star. We propose a multiband study of NGC 1313 X-2 from near-IR to X-rays, with XMM, HST and Swift. We will measure the contribution of X-ray reprocessing to the optical emission and determine whether irradiation correlates with disk winds.