| 其他摘要 | Gamma-ray burst (GRB) refers to a sudden burst of gamma rays from any direction in space in a short time. The isotropic energy released by GRBs in a few seconds can reach ∼ 1051 − 1054 erg, which is the most violent electricmagnetic explosion phenomenon in the universe discovered so far. Since 1967, the military satellite Vela of USA detected GRB from space the first, GRBs have been gradually understood by astronomers. Especially in recent years, many breakthroughs have been made in the field of GRBs. For example, in August 2017, for the first time, human beings detected the gravitational wave (GW170817) produced by the coalescence of binary neutron stars and found that it is associated with the short GRB (GRB 170817a). GRB 170817a is a unique short GRB with a low isotropic energy of Eγ,iso ∼ 6 × 1046 erg, and the light curve of the afterglow has a long slowly rising phase of ～160 days. In addition, great progress has been made in the polarization detection of GRBs. China's POLAR detected a GRB sample with low average polarization degree (～10%), and GRB 170114A was found with polarization angle evolution in a single pulse. The first chapter is foreword. The second and third chapters introduce the observation and theoretical models of GRBs. The second chapter mainly introduces the observation of prompt emission and afterglow of GRBs, including the spatial distribution and burst rate of GRBs, the observed light curve and spectrum in the prompt emission and afterglow phases, and the synchrotron radiation spectrum of the afterglow. In addition, the latest observations of very high-energy radiation from GRBs are shown in the last section of this chapter. In the second chapter, two main dynamic models of GRBs are introduced, including fireball model (dominated by baryon material) and Poynting flux dominated model. The introduction of the fireball model mainly includes the description of the model and the dynamic evolution of the fireball, as well as the characteristic radius of the fireball model. As the first GRB associated with gravitational waves, GRB 170817A's unique super-long and slowly rising afterglow light curves are difficult to be explained by the classical uniform top-hat jet model or the uniform circumburst medium. Therefore, the research of Chapter 4 and Chapter 5 of this paper will be dedicated to solve this problem. In chapter 4 and chapter 5, the circumburst environmental effect and the jet structure effect are studied respectively. We systematically study the afterglow light curves in different circumburst environment and different viewing angles in the fourth chapter. We obtain the analytical temporal indexes of off-axis observation under different circumburst environment by taking an analytical approximation, and we also give the corresponding light curves by taking the numerical calculation. Finally, the model is applied to five low luminosity GRBs to fit the afterglow data. We found that the light curves of four low luminosity GRBs, including GRB 170817A, can be well fitted under slightly off-axis observations, but the given circumburst environment is not exactly the same. Although the afterglow light curves of GRB 170817A can be well fitted in the environment with a burst medium distribution of n ∝ R−1.6, the parameters given by the fitting cannot meet the observation limits of radio afterglow images. Even so, we have learned from this work that a special afterglow light curve with a long and slowly rising phase can also be generated by a pure circumburst environmental effect. If similar afterglow light curves are observed in the future, we should consider the possible origin of the circumburst environment. The study of jet structure will help us to reveal the nature of the central engine, the mechanism of jet production and the radiation mechanism of GRBs. In Chapter 5, we explain the unique afterglow light curves of GRB 170817A from the perspective of jet structure effect. Different from the classical top-hat jet, we use a stratified jet model (collimated jet with radial structure) to fit the afterglow data of GRB 170817A, and further constrained the model parameters by the observed source size and the angular displacement of flux centroid of the radio afterglow image. In the stratified jet model, the long and slowly rising afterglow light curves are due to the radial structure of the jet, while the rapid decline of the late time afterglow flux is due to the edge effect of the jet. This suggests that not only for GRB 170817A, the stratified jet model should be considered as a possible candidate model if similar afterglow light curves are observed in other GRBs in the future. Studying the polarization of GRBs is of great significance for us to understand the jet structure, magnetic field (MF) configuration and the radiation mechanism of GRBs. In the sixth chapter, we study the polarization of GRB prompt emission, including the time-resolved and time-averaged polarization. Our calculation results show that the time-average polarization degrees of decaying MF (~0.6) is higher than that of constant MF (~0.5) under typical parameters. This provides an opportunity for us to distinguish the two magnetic models in observation. In the future, high-precision polarization detectors will be able to distinguish the two models. In addition, we also found that the polarization angle of GRBs will change by 90 degrees twice in a single pulse in the toroidal MF (decaying MF). Such a result is roughly consistent with the discovery of the PA evolution within a pulse in some bursts, such as GRB 170114A and GRB 160821A. However, considering that the current number of GRBs with similar polarization angle change is very small and the current precision of the detectors are not high enough, it is difficult to determine whether the polarization angle change of GRBs originated from the toroidal MF model. If more similar polarization angle changes are detected by high-precision polarization detectors in the future, and considering the Band-like spectrum given by the decaying MF, we may determine the MF structure and radiation mechanism of GRBs. In the seventh chapter, we summarize the research of this paper and discuss the current open questions and prospect in GRBs. |
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