  Attempts were made to construct a unified description of the spectra of ULX
(Ultra Luminous X-ray source) objects, including their Power-Law (PL) state and
Disk-like state. Among spectral models proposed to explain either state, the
present work adopts the one which combines multi-color disk (MCD) emission and
its thermal Comptonization (THC). This model was applied to several datasets of
ULXs obtained by Suzaku, XMM-Newton, and Nustar. The model well explains all
the spectra, regardless of the spectral states, in terms of a cool disk (inner
radius temperature of 0.2-0.5 keV) and a cool thick (electron temperature of
1-3 keV, and optical thickness ~10) corona. The fit results can be
characterized by two new parameters. One is Q (defined as the electron
temerature divided by the inner radius temperature) which describes balance
between the Compton cooling and gravitational heating of the coronal electrons,
while the other is F, namely, the covering fraction of the MCD by the corona.
Here, F is calculated from the percentage of the directly-visible disk
luminosity in the total radiation. Then, the PL-state spectra have been found
to show Q~10 and F~0.5, while those of the Disk-like state Q~3 and F~1. Thus,
the two states are clearly separated in terms of Q and F. The obtained results
are employed to argue for their interpretation in terms of high-mass (several
tens to several hundreds solar masses) black holes.
