GRB 120711A: an intense INTEGRAL burst with long-lasting soft γ-ray emission and a powerful optical flash
Abstract
A long and intense γ-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 γ-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 (AV ~ 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 γ-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 (Epeak ~ 1 MeV)
and yields an Eγ,iso ~
1054 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 γ-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 (ηγ = 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
γ-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
γ-ray
emission has recently been observed from the nearby and extremely bright
Fermi/LAT burst GRB 130427A.
Origin : Publication funded by an institution
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