Discovering the nature of bright ULXs with optical spectroscopy

There are several scenarios for the nature of ultraluminous X-ray sources: some may be X-ray binaries accreting in the super-Eddington regime, while some of the brightest ULXs may be accreting intermediate mass black holes or recoiling supermassive black holes. Observational evidence for IMBHs and recoiling SMBHs is scarce, and finding these objects will provide new insights into galaxy evolution and black hole growth. Optical spectroscopy can help us distinguish between the different scenarios and weed out contaminants such as background AGN. We selected 5 ULXs from the catalog of Walton et al. (2012) with X-ray luminosities > 10^{40} erg/s and clear optical counterparts and observed these with the FORS2 instrument on the VLT. We present the resulting spectra and our interpretation.