In the past few years the number of well-sampled optical to NIR light curves of long Gamma-Ray Bursts (GRBs) has greatly increased particularly due to simultaneous multi-band imagers such as GROND. Combining these densely sampled ground-based data se
ts with the Swift UVOT and XRT space observations unveils a much more complex afterglow evolution than what was predicted by the most commonly invoked theoretical models. GRB 100814A represents a remarkable example of these interesting well-sampled events, showing a prominent late-time rebrightening in the optical to NIR bands and a complex spectral evolution. This represents a unique laboratory to test the different afterglow emission models. Here we study the nature of the complex afterglow emission of GRB 100814A in the framework of different theoretical models. Moreover, we compare the late-time chromatic rebrightening with those observed in other well-sampled long GRBs. We analysed the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 m MPG/ESO telescope together with the X-ray and UV data detected by the instruments onboard the Swift observatory. The broad-band afterglow evolution, achieved by constructing multi-instrument light curves and spectral energy distributions, will be discussed in the framework of different theoretical models. We find that the standard models that describe the broad-band afterglow emission within the external shock scenario fail to describe the complex evolution of GRB 100814A, and therefore more complex scenarios must be invoked. [abridged]
In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel Gamma-Ray Burst Optical and Near-infrared Detector (GROND) at the
2.2m MPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario. The afterglow of GRB 100621A shows a very complex temporal as well as spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using the prescription of Vlasis et al. (2011) for a two-shell collision after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large ($A_V$ = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy.
Context. After the launch of the Swift satellite, the Gamma-Ray Burst (GRB) optical light-curve smoothness paradigm has been questioned thanks to the faster and better sampled optical follow-up, which has unveiled a very complex behaviour. This compl
exity is triggering the interest of the whole GRB community. The GROND multi-channel imager is used to study optical and near-infrared (NIR) afterglows of GRBs with unprecedented optical and near-infrared temporal and spectral resolution. The GRB 081029 has a very prominent optical rebrightening event and is an outstanding example of the application of the multi-channel imager to GRB afterglows. Aims. Here we exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT observations to study the nature of late-time rebrightenings that appear in the optical-NIR light-curves of some GRB afterglows. Methods. We analyse the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and the X-ray data obtained with the XRT telescope on board the Swift observatory. The multi-wavelength temporal and spectral evolution is discussed in the framework of different physical models. Results. The extremely steep optical and NIR rebrightening observed in GRB 081029 cannot be explained in the framework of the standard forward shock afterglow model. The absence of a contemporaneous X-ray rebrightening and the evidence of a strong spectral evolution in the optical-NIR bands during the rise suggest two separate components that dominate in the early and late-time lightcurves, respectively. The steepness of the optical rise cannot be explained even in the framework of the alternative scenarios proposed in the literature unless a late-time activity of the central engine is assumed.
We selected a sample of 33 Gamma Ray Bursts (GRBs) detected by Swift, with known redshift and optical extinction at the host frame. For these, we constructed the de-absorbed and K-corrected X-ray and optical rest frame light curves. These are modelle
d as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circum-burst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibilities, as late prompt emission produced by a long lived central engine with mechanisms similar to those responsible for the production of the standard early prompt radiation. Apart from flares or re-brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based in part on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first step towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X-ray afterglows in a coherent way, by a relatively simple scenario. Within this context it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic; why the steepening after the jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall-back material (i.e. proportional to t^{-5/3}), suggesting that this can be the reason why the central engine can be active for a long time.
