  During its strong outburst of 2015 June/July, the X-ray transient V404 Cygni
(= GS2023+338) was observed up to a level of 50 Crab in the hard X-ray domain.
We focus here on a particularly intense episode preceeding a definitive decline
of the source activity. We benefit from large signal-to-noise ratios to
investigate the source spectral variability on a timescale of 5 minutes. A
Hardness-Intensity study of three broad bands reveals clearly different
behaviors at low and high energy (below and above around 100 keV). In
particular, on two occasions, the source intensity varies by a factor of 3-4 in
amplitude while keeping the same spectral shape. On the other hand, at the end
of the major flare, the emission presents a clear anticorrelation between flux
and hardness. These behaviors strongly suggest the presence of two spectral
components related to emission processes varying in a largely independent way.
The first component (E < 100-150 keV) is classically identified with a
Comptonizing thermal electron population and requires either an unusual seed
photon population or a specific geometry with strong absorbing/reflecting
material. The second component is modeled by a cutoff power-law, which could
correspond to a second hotter Comptonizing population or another mechanism
(synchrotron, non-thermal Comptonization...). In the framework of such a model,
Hardness-Intensity and Flux-Flux diagrams clearly demonstrate that the source
evolution follows a well organized underlying scheme. They reveal unique
information about the Hard X-ray emission processes and connections between
them.
