SIMBAD references

We present the results of a search for optical identifications of ultraluminous X-ray sources (ULXs) in M51 by using mosaic images taken with the Hubble Space Telescope(HST) Advanced Camera for Surveys (ASC) in filters F435W (B), F555W (V), F814W (I), and F658N (Hα). Our sample, consisting of nine ULXs, is defined by analyzing the three Chandra observations of M51 performed in 2000 June, 2001 June, and 2003 August. We found that four ULXs have one or two candidates for counterparts, while two have multiple stars within their error circles. The other three have no candidate counterparts. Four ULXs are located near or in a star cluster, while others have no association with a cluster. These results indicate that the companion stars, environments, and origins of ULXs are probably heterogeneous.