Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni

Understand

After more than 26 years in quiescence, the black hole transient V404 Cyg went into a luminous outburst in June 2015, and additional activity was detected in late December of the same year.

• Here, we present an optical spectroscopic follow-up of the December mini-outburst, together with X-ray, optical and radio monitoring that spanned more than a month.
• Strong flares with gradually increasing intensity are detected in the three spectral ranges during the ~10 days following the Swift trigger.
• Our optical spectra reveal the presence of a fast outflowing wind, as implied by the detection of a P-Cyg profile (He I - 5876 A) with a terminal velocity of ~2500 km/s.