Optical studies of ultra-luminous X-ray sources in nearby galaxies
  Optical studies of ultra-luminous X-ray sources is an essential step in understanding the nature of these enigmatic sources, and in this paper we report our studies of five ULXs in NGC4559, NGC5194, NGC1313 and NGC628 observed with the Hubble Space Telescope and the 6.4 Magellan/Baade telescope. The ULX in NGC4559 is identified with four blue and red supergiants within a $0\farcs2$ error circle, in a star forming region that is younger than $10^{7.4}$ years. ULX-3 in NGC5194 is located on the rim of a star cluster with a few faint stars within the $0\farcs3$ error circle, though the secondary in this system is expected to be a low mass ($0.3M_\odot$) star below detection limits given its two hour period. ULX-5 in NGC5194 is located on a spiral arm with seven objects within the $0\farcs8$ error circle, which can be improved with future observations. Both ULXs in NGC5194 are in regions younger than $10^{7.8}$ years. The ULXs in NGC1313 and NGC628 was observed with the Baade telescope. The ULX in NGC1313 is identified with one object with R-I color bluer than the bluest stellar objects, indicative of non-thermal emission in the R band, or dramatic variability between two observations, or errors in measurements. The ULX in NGC628 is located in a bubble nebula and identified with a few extended objects that are probably young stellar clusters. Future observations with better spatial resolutions are needed to clarify the optical counterparts and their nature for the ULXs in NGC1313 and NGC628. 
Ji-Feng Liu, Joel N. Bregman, Patrick Seitzer, and Jimmy Irwin