Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni
  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. Nebular-like
spectra -- with an H_alpha equivalent width of ~500 A -- are also observed. All
these features are similar to those seen during the main June 2015 outburst.
Thus, the fast optical wind simultaneous with the radio jet is most likely
present in every V404 Cyg outburst. Finally, we report on the detection of a
strong radio flare in late January 2016, when X-ray and optical monitoring had
stopped due to Sun constraints.
