2009ApJ...696.1712F

We examined spectral evolution in ultraluminous X-ray sources (ULXs) with apparent luminosities of about 10⁴⁰ erg/s. Based on new results in this paper, and those reported in the literature, two common spectral behaviors were found. Some ULXs in starburst galaxies have varying luminosity (L) but remain in the hard state with power-law spectra and a constant, hard photon index (Γ). Other ULXs, such as NGC 5204 X-1, show a correlation between L and Γ. We interpret this L-Γ correlated phase as an intermediate state with hybrid properties from the thermal dominant and steep power-law states. When the spectra of NGC 5204 X-1 are fitted with a multicolor disk blackbody plus power-law model, the X-ray luminosity increases with the effective temperature of the accretion disk in a manner similar to that found in stellar-mass black hole X-ray binaries, suggesting that the emission arises from an accretion disk. The luminosity, disk size, and temperature suggest that NGC 5204 X-1 harbors a compact object more massive than stellar-mass black holes. In contrast, the disk model in IC 342 X-1 is ruled out because the luminosity decreases as the temperature increases; sources with such behavior may represent a class of objects with super-Eddington accretion. Also, we report a peculiar soft spectral feature from IC 342 X-2 and variability on a timescale of 20 ks from Holmberg II X-1. More observations are needed to test these results.