A long and intense \gamma -ray burst ( GRB ) was detected by INTEGRAL on July 11 2012 with a duration of \sim 115 s and fluence of 2.8 \times 10 ^ { -4 } erg cm ^ { -2 } in the 20 keV –- 8 MeV energy range .
GRB 120711A was at z \sim 1.405 and produced soft \gamma -ray emission ( > 20 keV ) for at least \sim 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 \sim 11.5 mag at \sim 126 s after the trigger , or \sim 9th magnitude when corrected for the host galaxy extinction ( A _ { V } \sim 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 \sim 0.9 Ms. We present a comprehensive temporal and spectral analysis of the long-lasting soft \gamma -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/NIR detections from Watcher , Skynet /PROMPT , GROND , and REM .
The prompt emission had a very hard spectrum ( E _ { peak } \sim 1 MeV ) and yields an E _ { \gamma, iso } \sim 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 \gamma -ray emission using the standard afterglow scenario , which indicates a forward shock origin .
The combination of data extending from the NIR 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 \sim 1.2 , suggesting a massive star progenitor ( i.e .
Wolf-Rayet ) with a mass-loss rate between \sim 10 ^ { -5 } – 10 ^ { -6 } M _ { \odot } yr ^ { -1 } depending on the value of the radiative efficiency ( \eta _ { \gamma } = 0.2 or 0.5 ) .
The analysis of the reverse and forward shock emission reveals an initial Lorentz factor of \sim 120 – 340 , a jet half-opening angle of \sim 2 ^ { \circ } – 5 ^ { \circ } , and a baryon load of \sim 10 ^ { -5 } – 10 ^ { -6 } M _ { \odot } consistent with the expectations of the fireball model when the emission is highly relativistic .
Long-lasting soft \gamma -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 \gamma -ray emission has recently been observed from the nearby and extremely bright Fermi /LAT burst GRB 130427A .