A Unified View of the Spectral States of ULXs Using the Te/Tin Ratio

Ultra-Luminous X-ray sources (ULXs) are unusually bright X-ray objects in external galaxies. Though still controversial, they are good candidates for intermediate mass black holes (Makishima+2000). They are found either in the Disk-like state wherein the spectra have round shapes, or in the Power-Law (PL) state with power-law shaped spectra. Although the former spectra are often fitted with a Slim disk model while the latter ones with a Comptonized disk model (Cdm), the former are also reproduced with the Cdm, leading to more reasonable interpretation (Miyawaki+2009). Aiming at a unified description of the two states, we applied the Cdm to the spectra of several ULXs observed by Suzaku. Regardless of the spectral states, the model was successful on every data set, yielding relatively cool disk with a sub-keV temperature T_{in}, and a corona with an electron temperature of T_{e} ∼ a few keV. To quantify the fitting results, we introduced a new parameter Q=T_{e}/T_{in}, which represents the balance between coronal cooling by photons and heating by ions. We found that Q successfully distinguish the two states of ULXs, with Q ∼ 3 for the Disk-like state and Q ∼ 10 for the PL state.