  The black hole binary GS 2023+338 exhibited an unprecedently bright outburst
on June 2015. Since June 17th, the high energy instruments on board INTEGRAL
detected an extremely variable emission during both bright and low luminosity
phases, with dramatic variations of the hardness ratio on time scales of
~seconds. The analysis of the IBIS and SPI data reveals the presence of hard
spectra in the brightest phases, compatible with thermal Comptonization with
temperature kTe ~ 40 keV. The seed photons temperature is best fit by kT0 ~ 7
keV, that is too high to be compatible with blackbody emission from the disk.
This result is consistent with the seed photons being provided by a different
source, that we hypothesize to be a synchrotron driven component in the jet.
During the brightest phase of flares, the hardness shows a complex pattern of
correlation with flux, with a maximum energy released in the range 40-100 keV.
The hard X-ray variability for E > 50 keV is correlated with flux variations in
the softer band, showing that the overall source variability cannot originate
entirely from absorption, but at least part of it is due to the central
accreting source.
