Spectroscopy of Optical Counterparts of Ultraluminous X-ray Sources

Abstract

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts held with the 6m SAO telescope. In two ULXNe we detected for the first time signatures of high excitation ([OIII]5007 / Hβ> 5). Two of the ULXs were identified with young (T ~ 5-10 Myr) massive star clusters. Four of the eight ULX Nebulae (ULXNe) show bright high-excitation lines. This requires existence of luminous (~ 10^{38} .. 10^{40} erg/s) UV/EUV sources coinciding with the X-ray sources. Other 4 ULXNe require shock excitation of the gas with shock velocities of 20-100km/s. However, all the studied ULXN spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with the luminosity at least ~10^{38} erg/s. 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 powering their nebulae with \~10^{39} erg/s. Both the wind/jet activity and the EUV source needed are consistent with the suggestion that ULXs are high-mass X-ray binaries with the supercritical accretion disks of the SS433 type.