Optical Counterpart to the Ultraluminous X-Ray Source in the UGC6456 Galaxy

Abstract: We report the identification of the optical counterpart to the transient ultraluminuos X-ray source in the blue dwarf galaxy UGC6456 (VII Zw 403). The source is highly variable in both the X-ray (more 100 times, 0.3-10 keV) and optical (more 3 times, V band) ranges. The peak X-ray luminosity of UGC6456 ULX exceeds $10^{40}$ erg/s; the absolute magnitude when the source is optically bright is M$_V = -8.24\pm0.11$, 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 $0.9\pm 0.3$), 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 UGC6456 ULX show broad and variable hydrogen and helium emission lines, which also confirms the presence of the strong wind.