Interpretation of gamma-ray burst X-ray and optical afterglow emission : From the central engine to the circumburst interaction

Gamma-ray bursts are the largest electromagnetic explosions known to happen in the Universe and are associated with the collapse of stellar progenitors into blackholes. After an energetic prompt emission phase, lasting typically less than a minute and emitted in the gamma-rays, a long-lived afterglow phase starts. During this phase strong emission is observed at longer wavelengths, e.g., in the X-ray and optical bands. This phase can last several weeks and carries important information about the energetics and structure of the burst as well as about the circumburst medium (CBM) and its density profile. The standard afterglow model includes a single emission component which comes from synchrotron emission in a blast wave moving into the CBM. Additional factors that could give observable features include prolonged energy injection from the central engine, effects of the jet geometry, and viewing angle effects, which thus constitute an extended standard model. In this thesis, I study the afterglow emission in a global approach by analysing large samples of bursts in search for general trends and characteristics. In paper I, I compare the light curves in the X-rays and in the optical bands in a sample of 87 bursts. I find that 62% are consistent with the standard afterglow model. Among these, only 9 cases have a pure single power law flux decay in all bands, and are therefore fully described by the model within the observed time window. Including the additional factors described above, I find that 91% are consistent with the extended standard model. An interesting finding is that in nearly half of all cases the plateau phase (energy injection phase) changes directly into the jet decay phase. In paper II, I study the afterglow by analysing the temporal evolution of color indices (CI), defined as the magnitude difference between two filters. They can be used to study the energy spectrum with a good temporal resolution, even when high-resolution spectra are not available. I find that a majority of the CI do not vary ...