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We propose the thick-disc model of Gu et al. 2016 to interpret the transitionbetween soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) statein NGC 247. As accretion rate increases, the inner disc will puff up and act asshield to block the innermost X-ray emission regions and absorb both soft andhard X-ray photons. The absorbed X-ray emission will be re-radiated as a muchsofter blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hardX-ray band and transits from the SUL state to the SSUL state. The $\sim 200$stransition timescale can be explained by the viscous timescale. According toour model, the inner disc in the super-soft state is thicker and has smallerviscous timescale than in the soft state. X-ray flux variability, which isassumed to be driven by accretion rate fluctuations, might be viscoustime-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable.The bolometric luminosity is saturated in the thick disc; the observedradius-temperature relation can therefore be naturally explained.

Thick-disc model to explain the spectral state transition in NGC 247