The optical counterpart of the ultraluminous X-ray source NGC 5204 X-1

Abstract

We use archival HST/WFPC2 V-and I-band images to show that the optical counterpart to the ultraluminous X-ray source NGC 5204 X-1, reported by Roberts et al., is composed of two sources separated by 0.5 arcsec. We have also identified a third source as a possible counterpart, which lies 0.8 arcsec from the nominal X-ray position. Point spread function fitting photometry yields V-band magnitudes of 20.3, 22.0 and 22.4 for the three sources. The V - I band colours are 0.6, 0.1 and -0.2, respectively (i.e. the fainter sources are bluer). We find that all V - I colours and luminosities are consistent with those expected for young stellar clusters (age <10 Myr).