Optical Counterpart to the Ultraluminous X-Ray Source in the UGC 6456 Galaxy

We report the identification of the optical counterpart to the transient ultraluminous X-ray source in the blue dwarf galaxy UGC 6456 (VII Zw 403). The source is highly variable in both the X-ray (by a factor of at least 100, 0.3–10 keV) and optical (by a factor of three, V-band) ranges. The peak X-ray luminosity of UGC 6456 ultraluminous X-ray source (ULX) exceeds 1040 erg s−1; the absolute magnitude when the source is optically bright is MV = −8.24 ± 0.11, which makes this source one of the brightest ULXs in the optical range. We found a correlation between the optical and X-ray fluxes (with a coefficient of 0.9 ± 0.3), which may indicate that the optical emission is produced by re-processing of the X-rays in outer parts of the optically thick wind coming from the supercritical accretion disk. Optical spectra of UGC 6456 ULX show broad and variable hydrogen and helium emission lines, which also confirms the presence of the strong wind.