GRB 120711A: an intense INTEGRAL burst with long-lasting soft<i>γ</i>-ray emission and a powerful optical flash

A long and intense -ray burst (GRB) was detected by INTEGRAL on 11 July 2012 with a duration of 115 s and fluence of 2.8 10 -4 erg cm -2 in the 20 keV-8 MeV energy range. GRB 120711A was at z 1.405 and produced soft -ray emission (>20 keV) for at least 10 ks after the trigger. The GRB was observed by several ground-based telescopes that detected a powerful optical flash peaking at an R-band brightness of 11.5 mag at 126 s after the trigger, or 9th magnitude when corrected for the host galaxy extinction (A V 0.85). The X-ray afterglow was monitored by the Swift, XMM-Newton, and Chandra observatories from 8 ks to 7 Ms and provides evidence for a jet break at 0.9 Ms. We present a comprehensive temporal and spectral analysis of the long-lasting soft -ray emission detected in the 20-200 keV band with INTEGRAL/IBIS, the Fermi/LAT post-GRB detection above 100 MeV, the soft X-ray afterglow and the optical/near-infrared detections from Watcher, Skynet/PROMPT, GROND, and REM. The prompt emission had a very hard spectrum (E peak 1 MeV) and yields an E ,iso 10 54 erg (1 keV-10 MeV rest frame), making GRB 120711A one of the most energetic GRBs detected so far. We modelled the long-lasting soft -ray emission using the standard afterglow scenario, which indicates a forward shock origin. The combination of data extending from the near-infrared to GeV energies suggest that the emission is produced by a broken power-law spectrum consistent with synchrotron radiation. The afterglow is well modelled using a stratified wind-like environment with a density profile k 1.2, suggesting a massive star progenitor (i.e. Wolf-Rayet) with a mass-loss rate between 10 -5 -10 -6 M yr -1 depending on the value of the radiative efficiency ( = 0.2 or 0.5). The analysis of the reverse and forward shock emission reveals an initial Lorentz factor of 120-340, a jet half-opening angle of 2 -5 , and a baryon load of 10 -5 -10 -6 M consistent with the expectations of the fireball model when the emission is highly relativistic. Long-lasting soft -ray emission from other INTEGRAL GRBs with high peak fluxes, such as GRB 041219A, was not detected, suggesting that a combination of high Lorentz factor, emission above 100 MeV, and possibly a powerful reverse shock are required. Similar long-lasting soft -ray emission has recently been observed from the nearby and extremely bright Fermi/LAT burst GRB 130427A.