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A long and intense gamma-ray burst (GRB) was detected by INTEGRAL on July 112012 with a duration of ~115s and fluence of 2.8x10^-4 erg cm^-2 in the 20keV-8 MeV energy range. GRB 120711A was at z~1.405 and produced soft gamma-rayemission (>20 keV) for at least ~10 ks after the trigger. The GRB was observedby several ground-based telescopes that detected a powerful optical flashpeaking at an R-band brightness of ~11.5 mag at ~126 s after the trigger. Wepresent a comprehensive temporal and spectral analysis of the long-lasting softgamma-ray emission detected in the 20-200 keV band with INTEGRAL, the Fermi/LATpost-GRB detection above 100 MeV, the soft X-ray afterglow from XMM-Newton,Chandra, and Swift and the optical/NIR detections from Watcher, Skynet, GROND,and REM. We modelled the long-lasting soft gamma-ray emission using thestandard afterglow scenario, which indicates a forward shock origin. Thecombination of data extending from the NIR to GeV energies suggest that theemission is produced by a broken power-law spectrum consistent with synchrotronradiation. The afterglow is well modelled using a stratified wind-likeenvironment with a density profile k~1.2, suggesting a massive star progenitor(i.e. Wolf-Rayet). The analysis of the reverse and forward shock emissionreveals an initial Lorentz factor of ~120-340, a jet half-opening angle of~2deg-5deg, and a baryon load of ~10^-5-10^-6 Msun consistent with theexpectations of the fireball model when the emission is highly relativistic.Long-lasting soft gamma-ray emission from other INTEGRAL GRBs with high peakfluxes, such as GRB 041219A, was not detected, suggesting that a combination ofhigh Lorentz factor, emission above 100 MeV, and possibly a powerful reverseshock are required. Similar long-lasting soft gamma-ray emission has recentlybeen observed from the nearby and extremely bright Fermi/LAT burst GRB 130427A.

GRB 120711A: an intense INTEGRAL burst with long-lasting soft gamma-ray emission and a powerful optical flash