Studies On High Energy Radiation Mechanisms And Gamma-Ray Burst Prompt Emissions

Gamma-ray Bursts (GRBs) are the most violent high-energy explosion in the universe. They are randomly happened, pulse-like phenomena with short durations. Since its discovery in1960’s by Vela satellites, GRBs have become a hot topic for astrophysical research. In1997the BeppoSAX satellite discovered afterglows of GRBs, and then helped measured GRB redshifts. Thus it was find that GRBs are events occured at cosmological distances. Now it is widely accepted that long bursts with durations longer than2s are from collapsing massive stars, while short bursts with durations are results of merging compact binaries. By studying GRBs the physical processes in ultrarelativistic and very high energy conditions can be investigated, and researches on other fields, including constraining cosmological models, can also get helped.The goal of this thesis is to present some discussions on possible radiation mechanisms and prompt light curves of GRBs. Since radiation mechanisms and prompt emissions are related to GRB central engines directly, studying these topics can help us to get a better understanding of some properties of the central engine. In Chapter1we reviewed the discovery and observa-tions of GRBs, presenting major achievements from major GRB-monitoring satellites including Compton Gamma-ray Observatory, BeppoSAX satellite, Swift satellite as well as the latest Fermi Gamma-ray Space Telescope. The multi-wavelength properties of prompt emission as well as afterglows of GRBs are also summarized in Chapter1.In Chapter2the current GRB standard model is presented. According to standard model, a fireball is ejected by the central engine. The internal shock is produced by collisions between various shells with different velocities inside the fireball. The directional kinetic energy of the fireball is then converted to internal energy, and finally non-thermal radiation by internal shocks, thus producing prompt emission. And the interaction between the fireball and the outer medium gives rise to external forward and reverse shocks, producing the observed afterglow. In the framework of standard model we introduced the properties of the fireball, the evolution of shocks, some possible radiation mechanisms as well as some post-standard effects. Also we present a brief introduction of central engines. And since the internal shock model for prompt emission has some problems hard to solve, we also introduce some possible alternatives, including the electromagnetic model, the turbulent model, as well as the the Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART) model.In Chapter3we present the spectrum of synchro-curvature self-Compton (SCSC) radia-tion of relativistic electrons with a power-law distribution of Lorentz factors. Synchro-curvature radiation from relativistic electrons moving around curved magnetic field lines and its self-Compton radiation are possible radiation mechanisms in the GRB enviroment. We find that the resulting SCSC spectrum is significantly different from that of either synchrotron self-Compton or curvature self-Compton radiation if both the curvature radius of the magnetic field and the cyclotron radius of the electrons are within some proper ranges. The effects of electrons’cool-ing and drifting, the low-energy self absorption in seed spectra, and the Klein-Nishina cutoff are also discussed, in order to get an accurate picture. We take gamma-ray bursts as our exam-ple environment for discussions. The results would be considered as a universal approach of the self-Compton emission of relativistic electrons moving in curved magnetic fields, and thus could be applied to many astrophysical phenomena, including active galactic nuclei (AGNs), pulsars, as well as GRBs.In Chapter4, we simulate the prompt emission light curves of GRBs within the framework of the ICMART model. This model applies to GRBs with a moderately-high magnetization parameter σ in the emission region. We show that this model can produce highly variable light curves with both fast and slow components. The rapid variability is caused by many locally Doppler-boosted mini-emitters due to turbulent magnetic reconnection in a moderately-high-σ ejecta. The run-away growth and subsequent depletion of these mini-emitters as a function time define a broad slow component for each ICMART event. A GRB light curve is usually composed of multiple ICMART events fundamentally driven by the erratic GRB central engine activity. Allowing variations of model parameters, including initial Lorentz factor, orientation and size distribution of reconnection regions, jet opening angle, radius of emission region, as well as observing energy band, one is able to reproduce a variety of light curves and the power density spectra (PDS) as observed. We also discussed the effects on PDS by these parameters.Finally in Chapter5we present our discussions, and list some remaining problems to be solved, as well as observing instruments to be commissioned in the future. We also present calculations of synchro-curvature radiation in Appendix A, and some discussions on ICMART model in Appendix B.