Optical counterparts of ultraluminous X-ray sources

We present the optical identification and characterization of counterparts of four objects previously cataloged as ultraluminous X-ray (ULX) sources. The objects were selected from the E. Colbert & A. Ptak catalog. The optical counterparts are identified as pointlike objects with magnitudes in the range of similar to 17-19. The optical spectra of three of the sources (IXO 32, 37, and 40) show the presence of emission lines typical of quasars. The position of these lines allows a precise estimation of their redshifts (2.769, 0.567, and 0.789 for IXO 32, 37, and 40, respectively). The fourth X-ray source, IXO 35, is associated with a red object that has a spectrum typical of an M star in our Galaxy. These identifications are useful for building clean samples of ULX sources, selecting suitable targets for future observations, and performing statistical studies on the different populations of X-ray sources.