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 ( T _ { in } = 0.2 \ - - 0.5 keV ) and a cool thick ( T _ { e } = 1 \ - - 3 keV , \tau \sim 10 ) corona .
The fit results can be characterized by two new parameters .
One is Q \equiv T _ { e } / T _ { in } which describes balance between the Compton cooling and gravitational heating of the coronal electrons , while the other is F \equiv 1 - F _ { direct } / F _ { total } , namely , the covering fraction of the MCD by the corona .
Here , F _ { direct } and F _ { total } are luminosity in the directly-visible disk emission and the total radiation , respectively .
Then , the PL-state spectra have been found to show Q \sim 10 and F \sim 0.5 , while those of the Disk-like state Q \sim 3 and F \sim 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 M _ { \odot } ) black holes .