Gamma-ray bursts and their X-ray and optical afterglow.

The aim of this PhD thesis is to study and characterize the optical and X-ray emission of the afterglows of gamma-ray bursts (GRBs). GRBs are the most powerful sources of electromagnetic radiation in the universe, with an isotropic luminosity that can reach values of 1E54 erg/s. The Swift satellite, launched in November 2004, opened a new era for the study and understanding of the phenomenon of GRBs, thanks to the rapid response of its narrow FOV instruments that allows the accurate localization of most GRBs and the more complete coverage of the GRB evolution. In the first part of my PhD I was involved in a comprehensive statistical analysis of the Swift X-ray light-curves (LCs) of GRBs, carried out in a model-independent way. Our sample is composed of the X-ray LCs of more than 650 GRBs observed by Swift from December 2004 to December 2010. For 437 GRBs the statistics were good enough to allow us to extract a spectrum to convert their count-rate LCs into flux LCs. For GRBs with a known redshift, also rest-frame luminosity LCs in the 0.3-30 keV band were computed. From the fit of these LCs, we obtained the values of the temporal slopes and break times of the continuum of the X-ray emission, since the used fitting procedure automatically discards the positive fluctuations (i.e. flares). Then, we computed the total fluences and energies, those of flares and differentiating between the components of the X-ray LCs. Thanks to this large sample of LCs, we could carry out a homogeneous analysis of GRBs in a common rest frame energy band (0.3-30 keV), investigating the intrinsic time scales and energetics of the different LC phases. In addition, we studied the properties of flares superimposed to the smooth X-ray decay. GRBs are classified as long and short, depending on the duration of the prompt emission (T90>2 s and T90<2 s, respectively); our sample of GRBs allowed us to investigate the possible differencies and similarities between these two classes, for example the nature of long and short GRBs and the emission mechanisms involved. Finally, we examined the possible relation between the X-ray and gamma-ray emission and we found the existence of a universal scaling involving two parameters of the prompt emission and one of the X-ray emission: the isotropic prompt emission energy (E_{γ,iso}), the peak energy (E_{pk}) and the isotropic X-ray energy (E_{X,iso}). The main idea of the project presented above is to study all quantities that characterize the X-ray data and to look for a link between prompt and afterglow emission. During this work, we realized that the optical data were very important for our understanding, adding information to investigate the GRB emission mechanisms and to study the environment properties. Therefore, in the second part of my PhD we carried out a systematic analysis of the optical data available in literature, collecting data from all the available sources. From the collected optical data, we determine the shapes of the optical LCs. Then, we modeled the optical/X-ray spectral energy distribution (SED), we studied the SED parameter distributions and we compared the optical and X-ray LC slopes and shapes. For 20% of GRBs the difference between the optical and X-ray slopes is consistent with 0 or 1/4 within uncertainties (we do not consider here the steep decay phase), but in the majority of cases (80%) the optical and X-ray afterglows show significantly different temporal behaviors. Interestingly, we found an indication that the onset of the forward shock in the optical LCs (initial peaks or shallow phases) could be linked to the presence of the X-ray flares. Indeed when there are X-ray flares the optical LC initial peak or plateau end occurs during the steep decay, instead if there are no X-ray flares or if they occur during the plateau, the optical initial peak or plateau end takes place during the X-ray plateau. This could link the prompt emission with the optical emission. The forward shock model cannot explain all the features of the optical (e.g. bumps, late re-brightenings) and X-ray (e.g. flares, plateaus) LCs. However, the synchrotron model is a viable mechanism for GRBs afterglow emission at late times. Further to the intrinsic spectrum of the afterglow, the SED analysis allows to study the properties of the GRB environment, by quantifying the amount of absorption at optical and X-ray wavelengths. The first is due to dust while the latter is mostly due to metals. Our analysis shows that the gas-to-dust ratios of GRBs are larger than the values calculated for the Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud assuming solar abundances.