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The black-hole binary V404 Cyg entered the outburst phase in June 2015 after26 years of X-ray quiescence, and with its behaviour broke the outburstevolution pattern typical of most black-hole binaries. We observed the entireoutburst with the Swift satellite and performed time-resolved spectroscopy ofits most active phase, obtaining over a thousand spectra with exposures fromtens to hundreds of seconds. All the spectra can be fitted with an absorbedpower law model, which most of the time required the presence of a partialcovering. A blue-shifted iron-Kalpha line appears in 10% of the spectratogether with the signature of high column densities, and about 20% of thespectra seem to show signatures of reflection. None of the spectra showed theunambiguous presence of soft disk-blackbody emission, while the observedbolometric flux exceeded the Eddington value in 3% of the spectra. Our resultscan be explained assuming that the inner part of the accretion flow is inflatedinto a slim disk that both hides the innermost (and brightest) regions of theflow, and produces a cold, clumpy, high-density outflow that introduces thehigh-absorption and fast spectral variability observed. We argue that the blackhole in V404 Cyg might have been accreting erratically or even continuously atEddington/Super-Eddington rates - thus sustaining a surrounding slim disk -while being partly or completely obscured by the inflated disk and its outflow.Hence, the largest flares produced by the source might not be accretion-drivenevents, but instead the effects of the unveiling of the extremely bright sourcehidden within the system.

Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion