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 \sim 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 kT _ { e } \sim 40 keV .
The seed photons temperature is best fit by kT _ { 0 } \sim 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 can not originate entirely from absorption , but at least part of it is due to the central accreting source .