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

Abstract

We propose the thick-disc model of Gu et al. 2016 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 $\sim 200$s transition timescale can be explained by the viscous timescale. According to our model, the inner disc in the super-soft state is thicker and has smaller viscous timescale 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.

arXiv comment4 pages, 3 figures, Accepted for publication in MNRAS

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• XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows
• Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1
• The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1
• Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1
• Probing extreme black-hole outflows on short timescales via high spectral-resolution X-ray imagers