The dependence of the estimated luminosities of ultraluminous X-ray sources on spectral models

Data from Chandra observations of 30 nearby galaxies were analyzed and 365 X-ray point sources were chosen whose spectra were not contaminated by excessive diffuse emission and not affected by photon pileup. The spectra of these sources were fitted using two spectral models (an absorbed power-law and a disk blackbody) to ascertain the dependence of estimated parameters on the spectral model used. It was found that the cumulative luminosity function depends on the choice of the spectral model, especially for luminosities > 1040 ergs s-1. A large number (similar to 80) of the sources have luminosities > 1039 ergs s-1 (ultraluminous X-ray sources) with indistinguishable average spectral parameters (inner disk temperature similar to 1 keV and/ or photon index Gamma similar to 2) with those of the lower luminosity ones. We identify four sources whose minimum luminosity exceeds 1040 ergs s-1. Their spectra are in general better represented by the disk blackbody model than the power-law one. These extremely luminous X-ray sources (ELXs) can be grouped into two distinct spectral classes. Two of them have an inner disk temperature of < 0.5 keV (supersoft), while the other two have temperatures greater than or similar to 1.3 keV (hard). The estimated inner disk temperatures of the supersoft ELXs are compatible with the hypothesis that they harbor intermediate-size black holes, which are accreting at similar to 0.5 times their Eddington luminosity. The radiative mechanism for hard ELXs seems to be inverse Comptonization, which in contrast to standard black holes systems is probably saturated.