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

GRB 060607A: A gamma-ray burst with bright asynchronous early X-ray and optical emissions

The early optical emission of the moderately high redshift (2 = 3.08) GRB 060607A shows a remarkable broad and strong peak with a rapid rise and a relatively slow power-law decay. It is not coincident with the strong early-time flares seen in the X-ray and gamma-ray energy bands. There is weak evidence for variability superposed on this dominant component in several optical bands that can be related to flares in high-energy bands. While for a small number of gamma-ray bursts (GRBs), well-sampled optical flares have been observed simultaneously with X-ray and gamma-ray pulses, GRB 060607A is one of the few cases where the early optical emission shows no significant evidence for correlation with the prompt emission. In this work we first report in detail the broad-band observations of this burst by Swift. Then by applying a simple model for the dynamics and the synchrotron radiation of a relativistic shock, we show that the dominant component of the early emissions in optical wavelengths has the same origin as the tail emission produced after the main gamma-ray activity. The most plausible explanation for the peak in the optical light curve seems to be the cooling of the prompt after the main collisions, shifting the characteristic synchrotron frequency to the optical bands. The fact that the early emission in X-ray does not show a steep decay, like what is observed in many other GRBs, is further evidence for slow cooling of the prompt shell within this GRB. It seems that the cooling process requires a steepening of the electron energy distribution and/or a break in this distribution at high energies. From simultaneous gamma-ray emission during the first flare, the behaviour of hardness ratio, and the lack of spectral features, we conclude that the X-ray flares are due to the collision of late shells rather than late reprocessing of the central engine activities. The sharp break in the X-ray light curve at few thousands of seconds after the trigger, is not observed in the infrared/optical/ultraviolet bands, and therefore cannot be a jet break. Either the X-ray break is due to a change in the spectrum of the accelerated electrons or the lack of an optical break is due to the presence of a related delayed response component

Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion

The black hole (BH) binary V404 Cyg entered the outburst phase in 2015 June after 26 yr of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most BH binaries. We observed the entire outburst with the Swift satellite and performed timeresolved spectroscopy of its most active phase, obtaining over a thousand spectra with exposures from tens to hundreds of seconds. All the spectra can be fitted with an absorbed power-law model, which most of the time required the presence of a partial covering. A blueshifted iron-Kα line appears in 10 per cent of the spectra together with the signature of high column densities, and about 20 per cent of the spectra seem to show signatures of reflection. None of the spectra showed the unambiguous presence of soft disc–blackbody emission, while the observed bolometric flux exceeded the Eddington value in 3 per cent of the spectra. Our results can be explained assuming that the inner part of the accretion flow is inflated into a slim disc that both hides the innermost (and brightest) regions of the flow, and produces a cold, clumpy, high-density outflow that introduces the high absorption and fast spectral variability observed. We argue that the BH in V404 Cyg might have been accreting erratically or even continuously at Eddington/super-Eddington rates – thus sustaining a surrounding slim disc – while being partly or completely obscured by the inflated disc and its outflow. Hence, the largest flares produced by the source might not be accretion-driven events, but instead the effects of the unveiling of the extremely bright source hidden within the system