We report on the discovery and properties of the fading afterglow and underlying host galaxy of GRB 071028B, thereby facilitating a detailed comparison between these two. Observations were performed with the Gamma-ray Burst Optical and Near-infrared
Detector at the 2.2 m telescope on the La Silla Paranal Observatory in Chile. We conducted five observations from 1.9 d to 227.2 d after the trigger and obtained deep images in the griz and JHKs bands. Based on accurate seven-channel photometry covering the optical to near-infrared wavelength range, we derive a photometric redshift of z = 0.94 +0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag is moderately absorbed and requires a relatively flat extinction curve. According to the reported Swift/BAT observations, the energetics yield an isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.
X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with the peak energy of the time-integrated spectrum, Ep, below 30 keV, whereas classical GRBs have Ep of a few hundreds keV. Apart from Ep and the lower luminosity, the properties of XRFs a
re typical of the classical GRBs. Yet, the nature of XRFs and the differences from that of GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. We examine in detail the case of XRF 080330 discovered by Swift at the redshift of 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich and broadband (from NIR to UV) photometric data set we collected across this phase makes it an ideal candidate to test the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. We present prompt gamma-ray, early and late IR/visible/UV and X-ray observations of the XRF 080330. We derive a SED from NIR to X-ray bands across the plateau phase with a power-law index of 0.79 +- 0.01 and negligible rest-frame dust extinction. The multi-wavelength evolution of the afterglow is achromatic from ~10^2 s out to ~8x10^4 s. We describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model and show that a single-component jet viewed off-axis explains the observations (abriged).
The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on the a
ccurate knowledge of the distance to the events. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at >0.1 GeV. Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5% most distant GRBs, and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is rather surprising. The observed gamma-ray variability in the prompt emission together with the redshift suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous measurements of Gamma in GRBs and strengthens the extreme nature of GRB 080916C.
We describe the construction of GROND, a 7-channel imager, primarily designed for rapid observations of gamma-ray burst afterglows. It allows simultaneous imaging in the Sloan griz and near-infrared $JHK$ bands. GROND was commissioned at the MPI/ESO
2.2m telescope at La Silla (Chile) in April 2007, and first results of its performance and calibration are presented.
