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

We propose the thick-disc model of Gu et al. to interpret the transition between soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) state in NGC 247. As accretion rate increases, the inner disc will puff up and act as shield to block the innermost X-ray emission regions and absorb both soft and hard X-ray photons. The absorbed X-ray emission will be re-radiated as a much softer blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hard X-ray band and transits from the SUL state to the SSUL state. The ∼200 s transition time-scale can be explained by the viscous time-scale. According to our model, the inner disc in the supersoft state is thicker and has smaller viscous time-scale than in the soft state. X-ray flux variability, which is assumed to be driven by accretion rate fluctuations, might be viscous time-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable. The bolometric luminosity is saturated in the thick disc; the observed radius–temperature relation can therefore be naturally explained. [ABSTRACT FROM AUTHOR]