Spectral States and Evolution of Ultraluminous X-ray Sources
  We examined spectral evolution in ultraluminous X-ray sources (ULXs) with
apparent luminosities of about 10^40 ergs/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 (Gamma).
Other ULXs, such as NGC 5204 X-1, show a correlation between L and Gamma. We
interpret this L-Gamma 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 behaviors 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 time scale
of 20 ks from Holmberg II X-1. More observations are needed to test these
results.
