High excitation emission line nebula associated with an ultra-luminous X-ray source at z = 0.027 in the AKARI North Ecliptic Pole Deep Field

Aims. We report our finding of a high excitation emission line nebula associated with an ultra-luminous X-ray source (ULX) at z = 0.027, which we found in our Chandra observation of the AKARI North Ecliptic Pole Deep Field. Methods. We present a Chandra X-ray and Gran Telescopio Canarias (GTC) optical spectral analysis of the ULX blob. We investigated the nature of the emission line nebula using line ratio diagnostic diagrams and estimated the physical properties of the nebula with spectral energy distribution (SED) fitting. Results. The optical spectrum of this ULX blob shows emission line ratios that are located on the borderlines between star formation and Seyfert regimes in [OIII]/H β -[OI]/H α , [OIII]/H β -[SII]/H α, and [OIII]/H β -[OIII]/[OII] diagnostic diagrams. These are in contrast with those of a nearby blob observed with the same slit, which occupies the HII regimes. Conclusions. This result suggests that the energy input from the accretion power of the ULX, in addition to the star formation activity in the blob, significantly contribute to the ionization of the emission line nebula associated with the ULX, suggesting the existence of an accretion disk in the ULX emitting UV radiation or exerting shock waves.