The early- and late-time spectral and temporal evolution of ORB 050716

We report on a comprehensive set of observations of gamma-ray burst 050716, detected by the Swift satellite and subsequently followed-up rapidly in X-ray, optical and near-infrared (NIR) wavebands. The prompt emission is typical of long-duration bursts, with two peaks in a time interval of T90= 68 s (15–350 keV). The prompt emission continues at lower flux levels in the X-ray band, where several smaller flares can be seen on the top of a decaying light curve that exhibits an apparent break around 220 s post-trigger. This temporal break is roughly coincident with a spectral break. The latter can be related to the extrapolated evolution of the break energy in the prompt γ-ray emission, and is possibly the manifestation of the peak flux break frequency of the internal shock passing through the observing band. A possible 3σ change in the X-ray absorption column is also seen during this time. The late-time afterglow behaviour is relatively standard, with an electron distribution power-law index of p= 2; there is no notable temporal break out to at least 10 d. The broad-band optical/NIR to X-ray spectrum indicates a redshift of z≳ 2 for this burst, with a host-galaxy extinction value of EB−V≈ 0.7 that prefers a small magellanic cloud (SMC)-like extinction curve

The circumburst environment of a FRED GRB: Study of the prompt emission and X-ray/optical afterglow of GRB 051111

Aims.We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB 051111 and discuss its properties in the context of current fireball models. Methods.The detection of GRB 051111 by the Burst Alert Telescope on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray Telescope. Results.The prompt $\gamma$-ray emission shows a classical FRED profile. The optical afterglow light curves are fitted with a broken power law, with ${\alpha}_1=0.35$ to ${\alpha}_2=1.35$ and a break time around 12 min after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between the $\gamma$-ray tail of the FRED temporal profile and the late XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the optical decay, this would represent one of the first GRBs for which the canonical steep-shallow-normal decay typical of early X-ray afterglows has been monitored optically. We present a detailed analysis of the intrinsic extinction, elemental abundances and spectral energy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some $\alpha$ elements such as oxygen, [O/Zn] = 0.7 $\pm$ 0.3, or, alternatively, for a significant presence of molecular gas. The IR-to-X-ray Spectral Energy Distribution measured at 80 min after the burst is consistent with the cooling break lying between the optical and X-ray bands. Extensive modelling of the intrinsic extinction suggests dust with big grains or grey extinction profiles. The early optical break is due either to an energy injection episode or, less probably, to a stratified wind environment for the circumburst medium

Anatomy of a dark burst - The afterglow of GRB 060108

We present a multiwavelength study of GRB 060108 – the 100th gamma-ray burst discovered by Swift. The X-ray flux and light curve (three segments plus a flare) detected with the X-ray Telescope are typical of Swift long bursts. We report the discovery of a faint optical afterglow detected in deep BVRi′-band imaging obtained with the Faulkes Telescope North beginning 2.75 min after the burst. The afterglow is below the detection limit of the Ultraviolet/Optical Telescope within 100 s of the burst, while is evident in K-band images taken with the United Kingdom Infrared Telescope 45 min after the burst. The optical light curve is sparsely sampled. Observations taken in the R and i′ bands can be fitted either with a single power-law decay in flux, F(t) ∝t−α where α= 0.43 ± 0.08, or with a two-segment light curve with an initial steep decay α1 < 0.88 ± 0.2, flattening to a slope α2∼ 0.31 ± 0.12. A marginal evidence for rebrightening is seen in the i′ band. Deep R-band imaging obtained ∼12 d post-burst with the Very Large Telescope reveals a faint, extended object (R∼ 23.5 mag) at the location of the afterglow. Although the brightness is compatible with the extrapolation of the slow decay with index α2, significant flux is likely due to a host galaxy. This implies that the optical light curve had a break before 12 d, akin to what observed in the X-rays. We derive the maximum photometric redshift z < 3.2 for GRB 060108. We find that the spectral energy distribution at 1000 s after the burst, from the optical to the X-ray range, is best fitted by a simple power law, Fν∝ν−β, with βOX= 0.54 and a small amount of extinction. The optical to X-ray spectral index (βOX) confirms GRB 060108 to be one of the optically darkest bursts detected. Our observations rule out a high redshift as the reason for the optical faintness of GRB 060108. We conclude that a more likely explanation is a combination of an intrinsic optical faintness of the burst, a hard optical to X-ray spectrum and a moderate amount of extinction in the host galaxy

Multi-wavelength afterglow observations of the high redshift GRB 050730

Context.GRB 050730 is a long duration high-redshift burst (z=3.967) that was discovered by Swift. The afterglow shows variability and was well monitored over a wide wavelength range. We present comprehensive temporal and spectral analysis of the afterglow of GRB 050730 including observations covering the wavelength range from the millimeter to X-rays. Aims.We use multi-wavelength afterglow data to understand the complex temporal and spectral decay properties of this high redshift burst. Methods.Five telescopes were used to study the decaying afterglow of GRB 050730 in the B, V, r', R, i', I, J and K photometric pass bands. A spectral energy distribution was constructed at 2.9 h post-burst in the B, V, R, I, J and K bands. X-ray data from the satellites Swift and XMM-Newton were used to study the afterglow evolution at higher energies. Results.The early afterglow shows variability at early times and the slope steepens at 0.1 days (8.6 ks) in the B, V, r', R, i', I, J and K passbands. The early afterglow light curve decayed with a powerlaw slope index $\alpha_1 = -0.60\pm0.07$ and subsequently steepened to $\alpha_2 = -1.71\pm0.06$ based on the R and I band data. A millimeter detection of the afterglow around 3 days after the burst shows an excess in comparison to theoretical predictions. The early X-ray light curve observed by Swift is complex and contains flares. At late times the X-ray light curve can be fit by a powerlaw decay with $\alpha_x = -2.5\pm0.15$ which is steeper than the optical light curve. A spectral energy distribution (SED) was constructed at ~2.9 h after the burst. An electron energy index, p, of ~2.3 was calculated using the SED and the photon index from the X-ray afterglow spectra and implies that the synchrotron cooling frequency $\nu_{\rm c}$ is above the X-ray band