| Gamma-Ray Bursts (GRBs) are short and intense unpredictable flashes ofγ-ray photons, emanating from seemingly random places in deep space at random times. For the most part they started with a "burst" of short-termγray photons and then developed into a longer-term "afterglow" emitting at longer electromagnetic wavelengths, such as X-rays, ultraviolet, optical, infrared, and radio. In the majority, they would take a few months to fade out, and then got completely removed from the ken of telescopes.In§1, we briefly reviewed the periodic progresses (Vela, BATSE/CGRO, Beppo-SAX&HETE-2) on the GRB observations since its first discovery; described the break-through results of new features (especially the shallow decay and X-ray flare) in X-ray afterglows detected in Swift era; presented and compared different models tempting to unveil problems tangled us ever before Swift launched; and shown the urgent demand of new generation of detectors.In§2, we overreviewed the mission as well as observational progresses and theo-retical suggestions for GRB high energy emissions detected by FGST/GLAST, after its operation for nearly 3 years up today. By comparing the positives and negatives of the existed models for some novel properties (LAT lag, extra component and curvature), revolutionized hypothesis is required; meanwhile, the confirmation of such properties' popularity needs larger level of statistic data, therefore impetus our further research.In§3, for the first time, we proposed a "structured outflow" model, aiming to pro-vide our interpretation to the LAT lag. By fitting with broadband afterglow light curves and spectrum of three long-duration gamma-ray bursts (GRBs 080916C,090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large Area Tele-scope (LAT) instruments on the Fermi satellite, we study the origin of the high energy emission, and suggest that the early-time GeV emission together with the late-time ra-dio, optical, and X-ray afterglows can be understood as being due to synchrotron emis-sion from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. Moreover, different origin of the high energy photons and lower energy ones can fasten the constraint of lower Lorentz factor, because sub-GeV photons only play the role as the background GeV ones, which is more natural than the ultra-relativistic jet (with bulk Lorentz factor > 1000) before. In particular, from our fits to observational data, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff effect is significant. Such an environment might result from superbub-bles or low-metallicity progenitor stars (which have a low mass-loss rate at late times of stellar evolution) of type Ib/c supernovae. This work received prompt interest among the scientific community after been submitted.In§4, we presented some other high energy transient phenomenons besides GRBs, detected by Fermi LAT or/and GBM detector after its coming into services. These new results also challenged the former interpretations, inspired out new interests, therefore fresh hypothesis making each consistent will undoubt significantly contribute to high energy particle physics and astrophysics understanding.Finally, in§5, we will briefly summarize the main contents and advantages in this thesis and propose the potential issue on structured ejecta in our work for the further research. We will discuss the open questions still exist in the Fermi era. Finally, we share a little thought and expectation for my Ph.D. Programme.
|
PDF Full Text Request