Comprehensive X-ray and multiwavelength study of ULXs in NGC 1566

This paper presents a comprehensive X-ray and multiwavelength study of ultraluminous X-ray sources (ULXs) in NGC 1566 using archival Chandra, Swift/XRT, JWST, and 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 were observed. Secondly, a new transient ULX candidate (ULX-4) was identified, reaching a peak luminosity of ~10⁴⁰ erg s^-1. Thirdly, 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 disc/dust disrupted by X-rays. Lastly, the optical observations suggest that the possible donor star of ULX-3 is a B-type supergiant. In the case of ULX-4, the multiwavelength properties are not clear since many sources are detected within the astrometric error radius.