2023MNRAS.526.5765A

This paper presents the identification of a new transient ultraluminous X-ray source (ULX) candidate (ULX-3) with reaching a peak luminosity of ∼4 × 10³⁹ erg s^–1 in NGC 4254 by using archival Chandra, Swift/X-Ray Telescope, Hubble Space Telescope, and JWST observations. From precise astrometric calculations, unique optical, near-infrared, and mid-infrared counterparts were found. The spectral energy distribution (SED) and colour-magnitude diagrams of counterparts of the new ULX candidate were plotted to constrain the nature of the possible donor star. Evidence of a circumbinary disc was found from its SED with two blackbody temperatures of 1000 and 200 K. Moreover, according to the X-ray hardness ratios, ULX-3 exhibits very hard to very soft transitions as seen in some high-mass X-ray binaries with Be-star donors. Moreover, ULX-3 varies by more than two orders of magnitude in the 0.3-10 keV energy band as seen in typical transient ULXs.