The ultraluminous compact X-ray sources ( ULXs ) generally show a curving spectrum in the 0.7–10 keV ASCA bandpass , which looks like a high temperature analogue of the disk dominated high/soft state spectra seen in Galactic black hole binaries ( BHBs ) at high mass accretion rates .
Several ULXs have been seen to vary , and to make a transition at their lowest luminosity to a spectrum which looks more like a power law .
These have been previously interpreted as the analogue of the power law dominated low/hard state in Galactic BHBs .
However , the ULX luminosity at which the transition occurs must be at least 10–50 per cent of the Eddington limit assuming that their highest luminosity phase corresponds to the Eddington limit , while for the Galactic BHBs the high/soft–low/hard transition occurs at a few per cent of the Eddington limit .
Here we show that the apparently power law spectrum in a ULX in IC342 can be equally well fit over the ASCA bandpass by a strongly Comptonised optically thick accretion disk with the maximum temperature of \sim 1 keV .
Recent work on the Galactic BHBs has increasingly shown that such components are common at high mass accretion rates , and that this often characterises the very high ( or anomalous ) state .
Thus we propose that the power law type ULX spectra are not to be identified with the low/hard state , but rather represent the Comptonisation dominated very high/ anomalous state in the Galactic BHBs .