The Dependence of the Estimated Luminosities of Ultraluminous X‐Ray Sources on Spectral Models

Abstract

Data from Chandra observations of 30 nearby galaxies were analyzed and365 X-ray point sources were chosen whose spectra were not contaminatedby excessive diffuse emission and not affected by photon pileup. Thespectra of these sources were fitted using two spectral models (anabsorbed power-law and a disk blackbody) to ascertain the dependence ofestimated parameters on the spectral model used. It was found that thecumulative luminosity function depends on the choice of the spectralmodel, especially for luminosities {\gt}10^{40} ergss^{-1}. A large number (~80) of the sources have luminosities{\gt}10^{39} ergs s^{-1} (ultraluminous X-ray sources)with indistinguishable average spectral parameters (inner disktemperature ~1 keV and/or photon index {Γ}~2) with those of thelower luminosity ones. We identify four sources whose minimum luminosityexceeds 10^{40} ergs s^{-1}. Their spectra are ingeneral better represented by the disk blackbody model than thepower-law one. These extremely luminous X-ray sources (ELXs) can begrouped into two distinct spectral classes. Two of them have an innerdisk temperature of {\lt}0.5 keV (supersoft), while the other two havetemperatures {\gt}~1.3 keV (hard). The estimated inner disk temperaturesof the supersoft ELXs are compatible with the hypothesis that theyharbor intermediate-size black holes, which are accreting at ~0.5 timestheir Eddington luminosity. The radiative mechanism for hard ELXs seemsto be inverse Comptonization, which in contrast to standard black holessystems is probably saturated.