Another interpretation of the power-law-type spectrum of an ultraluminous compact X-ray source in IC 342

Abstract

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 disc-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 IC 342 can be equally well fitted over the ASCA bandpass by a strongly Comptonized optically thick accretion disc with a maximum temperature of ∼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 characterizes 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 Comptonization-dominated very high/anomalous state in the Galactic BHBs.