High Excitation Emission Line Nebula associated with an Ultra Luminous
  X-ray Source at $z=$ 0.027 in the \textit{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 (NEP) Deep
Field. Methods. We present a Chandra X-ray and Gran Telescopio Canarias (GTC)
optical spectral analysis of the ULX blob. We investigate the nature of the
emission line nebula by using line ratio diagnostic diagrams, and its physical
properties estimated from 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$\beta$-[OI]/H$\alpha$, [OIII]/H$\beta$-[SII]/H$\alpha$ and
[OIII]/H$\beta$-[OIII]/[OII] diagnostic diagrams. These are in contrast with
those of a nearby blob observed with the same slit, which occupy the HII
regimes. This result suggests that the ionization of the emission line nebula
associated with the ULX is significantly contributed by energy input from the
accretion power of the ULX, in addition to the star formation activity in the
blob, suggesting the existence of an accretion disk in the ULX emitting UV
radiation, or exerting shock waves.
