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

The M101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ~ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources (ULXs) are still controversial. We re-examined the X-ray spectral and timing properties of the M101 ULS using archival Chandra and XMM-Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard disk emission. The characteristic radius R_{bb} of the thermal emitter varies from epoch to epoch between ~10,000 km and ~100,000 km; the colour temperature kT_{bb} varies between ~50 eV and ~140 eV; and the two quantities scale approximately as R_{bb} ~ T_{bb}^{-2}. In addition to the smooth continuum, we also find (at some epochs) spectral residuals well fitted with thermal plasma models and absorption edges: we interpret this as evidence that we are looking at a clumpy, multi-temperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the super-critical, radiatively driven outflow becomes effectively optically thick and completely thermalizes the harder X-ray photons from the inner part of the inflow, removing the hard spectral tail. We develop a simple, spherically symmetric outflow model and show that it is consistent with the observed temperatures, radii and luminosities. A larger, cooler photosphere shifts the emission peak into the far-UV and makes the source dimmer in X-rays but possibly ultraluminous in the UV. We compare our results and interpretation with those of Liu et al. (2013).