A new characterization of the Compton process in the ULX spectra

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, τ ∼10) corona. The fit results can be characterized by two new parameters. One is Q≡ T_e/T_in which describes the balance between the Compton cooling and gravitational heating of the coronal electrons, while the other is F≡ 1-F_direct/F_total, namely, the covering fraction of the MCD by the corona. Here, F_direct and F_total are the luminosity in the directly-visible disk emission and the total radiation, respectively. 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 M_⊙) black holes.