  This paper presents a comprehensive X-ray and multi-wavelength study of
ultraluminous X-ray sources (ULXs) in NGC 1566 using archival Chandra,
Swift/XRT, James Webb Space Telescope, JWST, and Hubble Space Telescope, HST
observations. The main results are, first, from the hardness ratio diagram,
where spectral state transitions from hard to soft as seen in typical Galactic
high mass X-ray binaries for ULX-3 was observed. Second, a new transient ULX
candidate (ULX-4) was identified, reaching a peak luminosity of 10E40 erg/s.
Third, the optical and NIR (near-infrared) counterparts of the ULXs were
searched from the precise astrometric calculations. For ULX-1 and ULX-2,
evidence was found that the observed NIR emission is due to the circumbinary
disk/dust disrupted by X-rays. Fourth, the optical observations suggest that
the possible donor star of ULX-3 is a B-type supergiant. In the case of ULX-4,
the multi-wavelength properties are not clear since many sources are detected
within the astrometric error radius.
