The topic of this PhD-thesis is Gamma-ray bursts (GRBs). GRBs are short, energetic bursts of gamma-rays. They, and their softer cousins the X-ray flashes (XRFs), are the manifestations of the most violent, cataclysmic explosions in the Universe. GRBs are followed by so-called afterglow emission detected in lower energy bands and on longer timescales, e.g. X-ray, UV, optical, nearinfrared, radio emissions from a few hundred seconds to a few months.
The first chapter of the thesis introduces the fundamental properties in the GRB prompt and afterglow phases and the most promising progenitor models for GRBs. A brief history on GRB research is also given in this chapter.
The second chapter introduces the GRB afterglow temporal and spectral evolution within the jetted internal-external shock model. This is the main foundation on which interpretation of all observed GRB afterglows rest.
The third chapter presents the detailed study of the multiwavelength afterglow of XRF060218 associated with the energetic supernova SN2006aj. Through this study, we can clearly see that the X-ray and optical afterglow emission of XRF060218 must originate from different physical processes. This is in contrast with the standard afterglow model and indicates that the X-ray afterglow could be attributed to a continued activity of the central engine that within the collapsar scenario could arise from fall-back accretion.
The fourth chapter focusses on the short GRB051221A. The afterglow of this burst provides strong evidence for energy injection from the central engine. This interesting phenomenon suggests that the postmerger object for some short GRBs could be magnetars.
The fifth chapter addresses the famous SN-less GRB 060505 and GRB 060614. These two events thereby challenge the conventional GRBSN connection for long GRBs. By investigating the two afterglows, we find that both afterglows can be well interpreted within the framework of the jetted standard external shock-wave model, and that the afterglow parameters for both bursts fall well within the range observed for other GRBs. Hence, from the properties of the afterglows there is nothing to suggest that these bursts should have another progenitor than other GRBs. Recently, GRB 080503 also had the spike + tail structure during its prompt gamma-ray emission seemingly similar to GRB 060614. We also analyse the prompt emission of this burst and find that this GRB is a hard-spike + hard-tail burst with a spectral lag of 0.8±0.4 s during its tail emission. Thus, the properties of the prompt emission of GRB 060614 and GRB 080503 are clearly different, showing that further thinking of the criteria for GRB classification is required. We also note that, whereas the progenitor of the two SN-less bursts remains uncertain, the core-collapse origin for the SN-less bursts would be quite certain if a wind-like environment can be observationally established, e.g, from an optical decay faster than the X-ray decay in the afterglows slow cooling phase.
The sixth chapter deals with the X-ray transient 080109 associated with SN 2008D. This serendipitous discovery may bring important new insight into both GRBs, XRFs and their link to Type Ibc supernovae (SNe). We propose that such X-ray transient, besides being caused by the shock breakout, might be caused by the interaction between a mildly relativistic jet and the surrounding wind medium, thus bridges the gap between energetic hypernovae and ordinary Type Ibc SNe. The thesis ends with an conclusion and outlook.
