New Hubble Space Telescope imaging of the counterparts to six   ultraluminous X-ray sources
  We report the results of new Hubble Space Telescope imaging of the positions of six ultraluminous X-ray sources (ULXs). Using images in three ACS filters we detect good candidate counterparts to four out of six ULXs, with one more possible detection, and observed magnitudes in the range m ~ 22 - 26 in the F606W filter. The extinction-corrected colours and absolute magnitudes vary from source to source, even after correcting for additional extinction in the host galaxy, and only one counterpart is readily explained as an OB star. Nevertheless, these counterparts are decent candidates for future follow-up in pursuit of dynamical mass constraints on the likely black holes powering these sources. 
T. P. Roberts (1), A. J. Levan (2), M. R. Goad (3) ((1) Durham   University, (2) University of Warwick, (3) University of Le...