Spectroscopy of optical counterparts of ultraluminous X-ray sources

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts performed with the 6m SAO telescope. In two ULX nebulae (ULXNe) we detected for the first time signatures of high excitation ([O III]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ∼ 5–10 Myr) massive star clusters. Four of the eight ULXNe show bright high-excitation lines. This requires existence of luminous (∼ 1038 ÷ 1040 erg s−1) UV/EUV sources coinciding with the X-ray sources. The other 4 ULXNe require shock excitation of the gas with shock velocities of 20–100 km s−1. However, all the studied ULXNe spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with a luminosity of at least ∼ 1038 erg s−1. Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups: shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets (∼ 1039 erg s−1) for powering their nebulae. Both the wind/jet activity and the existence of a bright UV source are consistent with the suggestion that ULXs are high-mass X-ray binaries with supercritical accretion disks of the SS433 type.