XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte-Carlo Simulated Model

We present a systematic spectral analysis of six ultraluminous X-ray sources (NGC1313 X-1/X-2, IC342 X-1, HoIX X-1, NGC5408 X-1 and NGC3628 X-1) observed with XMM-Newton Observatory. These extra-nuclear X-ray sources in nearby late-type galaxies have been considered as intermediate-mass black hole candidates. We have performed Monte-Carlo simulations of Comptonized multi-color black-body accretion disks. This unified and self-consistent spectral model assumes a spherically symmetric, thermal corona around each disk and accounts for the radiation transfer in the Comptonization. We find that the model provides satisfactory fits to the XMM-Newton spectra of thesources. The characteristic temperatures of the accretion disks (T_in), for example, are in the range of ~ 0.05-0.3 keV, consistent with the intermediate-mass black hole interpretation. We find that the black hole mass is typically about a few times 10^3 M_\odot and has an accretion rate ~ 10^{-6} - 10^{-5} M_\odot yr^{-1}. For the spectra considered here, we find that the commonly used multi-color black-body accretion disk model with an additive power law component, though not physical, provides a good mathematical approximation to the Monte-Carlo simulated model. However, the latter model provides additional constraints on the properties of the accretion systems, such as the disk inclination angles and corona optical depths.