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

Understand

The black-hole binary V404 Cyg entered the outburst phase in June 2015 after 26 years of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most black-hole binaries.

• We observed the entire outburst with the Swift satellite and performed time-resolved spectroscopy of its most active phase, obtaining over a thousand spectra with exposures from tens to hundreds of seconds.
• All the spectra can be fitted with an absorbed power law model, which most of the time required the presence of a partial covering.
• A blue-shifted iron-Kalpha line appears in 10% of the spectra together with the signature of high column densities, and about 20% of the spectra seem to show signatures of reflection.