A search for optical counterparts of ULX-4 in M51 galaxy

We present optical identification of counterparts of ULX-4 in spiral galaxy M51. Ultraluminous X-ray sources (ULXs) are variable, non-nuclear point-like X-ray sources with isotropic luminosities (Lx > 10³⁹ erg s^-1) above the Eddington limit for a 10 M_ʘ black hole. The ULX-4 has observed with Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) in filters F438W (B), F555W (V) and F814W (I), their data are available in archive. Two optical sources have been identified as counterpart candidates within the error radius of 0.2 arcsec and their V magnitudes are ∼23. The possible counterparts of ULX-4 were determined as B type supergiant stars, indicating that this system may be a high-mass X-ray binary.