The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?

We present a wide dataset of -ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814 the best

The nature of the outflow in gamma-ray bursts

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt γ-ray emission to the afterglow phase. The early-time X-ray and optical data for GRBs obtained by telescopes aboard the Swift satellite show that the source for prompt γ-ray emission, the emission that heralds these bursts, is short lived, and is distinct from the source for the long-lived afterglow emission that follows the initial burst. Using these data we determine the distance of the γ-ray source from the centre of the explosion. We find this distance to be 1015–1016 cm for most bursts, and show that this is within a factor of about 10 of the radius of the shock heated circumstellar medium (CSM) producing the X-ray photons. Furthermore, using the early γ-ray, X-ray and optical data we show that the prompt gamma-ray emission cannot be produced in internal shocks nor can it be produced in the external shock; in a more general sense γ-ray generation mechanisms based on shock physics have problems explaining the GRB data for ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has a number of attractive features, although evidence in its favour is inconclusive. Finally, the X-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons cm−3 at a distance of ∼5 × 1016 cm from the centre of explosion

GRB 090618: Detection of thermal X-ray emission from a bright gamma-ray burst

GRB 090618 was an extremely bright burst, detected across the electromagnetic spectrum. It has a redshift of 0.54 and a supernova (SN) was identified in ground-based photometry. We present a thorough analysis of the prompt and early afterglow emission using data from Swift, Fermi Gamma-ray Burst Monitor and ROTSE, in which we track the evolution of the synchrotron spectral peak during the prompt emission and through the steep decay phase. We find evidence of a thermal X-ray component alongside the expected non-thermal power-law continuum. Such a component is rare among gamma-ray bursts (GRBs), with firm data for only GRBs 060218 and 100316D so far, and could potentially originate from an SN shock breakout, although there remains doubt regarding this explanation for any of the bursts. However, in contrast to these other Swift GRB–SNe with similar thermal signatures, GRB 090618 is a much more ‘typical’ burst: GRB–SNe 060218 and 100316D were both low-luminosity events, with long durations and low peak energies, while GRB 090618 was more representative of the wider population of long GRBs in all of these areas. It has been argued, based both on theory and observations, that most long GRBs should be accompanied by SNe. If this thermal X-ray component is related to the SN, its detection in GRB 090618, a fairly typical burst in many ways, may prove an important development in the study of the GRB–SN connection

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