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The M101 galaxy contains the best-known example of an ultraluminous supersoftsource (ULS), dominated by a thermal component at kT ~ 0.1 keV. The origin ofthe thermal component and the relation between ULSs and standard (broad-bandspectrum) ultraluminous X-ray sources (ULXs) are still controversial. Were-examined the X-ray spectral and timing properties of the M101 ULS usingarchival Chandra and XMM-Newton observations. We show that the X-raytime-variability and spectral properties are inconsistent with standard diskemission. The characteristic radius R_{bb} of the thermal emitter varies fromepoch to epoch between ~10,000 km and ~100,000 km; the colour temperaturekT_{bb} varies between ~50 eV and ~140 eV; and the two quantities scaleapproximately as R_{bb} ~ T_{bb}^{-2}. In addition to the smooth continuum, wealso find (at some epochs) spectral residuals well fitted with thermal plasmamodels and absorption edges: we interpret this as evidence that we are lookingat a clumpy, multi-temperature outflow. We suggest that at sufficiently highaccretion rates and inclination angles, the super-critical, radiatively drivenoutflow becomes effectively optically thick and completely thermalizes theharder X-ray photons from the inner part of the inflow, removing the hardspectral tail. We develop a simple, spherically symmetric outflow model andshow that it is consistent with the observed temperatures, radii andluminosities. A larger, cooler photosphere shifts the emission peak into thefar-UV and makes the source dimmer in X-rays but possibly ultraluminous in theUV. We compare our results and interpretation with those of Liu et al. (2013).

Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model