GRB 120711A: an intense INTEGRAL burst with long-lasting softγ-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 (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 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 (E peak ∼ 1 MeV) and yields an E γ,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 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 (η γ = 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.

Fermi Observations of the Bright LAT GRB 160625B

“…The system started observing its error box at T 0 + 52 seconds after GBM and 136 seconds before the LAT trigger. The bright optical counterpart of this source is more consistent with GRB 120711A, which was also observed by several ground-based telescopes that detected a powerful optical flash peaking at an R-band brightness of ∼11.5 mag [15]. RATIR Optical/NIR [14] and MITSuME/Ishigakijima Optical [16] also detected an optical afterglow emission.…”

The Environments of the Most Energetic Gamma-Ray Bursts

“…An important point to clarify is that this need for an intrinsic error is driven primarily by bursts with very good datasets (see e.g. Ackermann et al 2013;Martin-Carrillo et al 2014;Perley et al 2014). These bursts have spectral and temporal indices that are well sampled over long baselines, and hence have small associated errors.…”

Optical observations of Very High Energy Sources from the Boyden Observatory

“…Recent observations with Watcher of GRB 120711A were reported by [6]. Time not used for GRB observations is used to observe, in particular, blazar sources.…”

“…6 Differential photometry was undertaken following a weighted mean approach [13] where each comparison has been weighted with the mean error (σ c,i ) associated with the comparison star over all N frames,…”