The Theory Of Early Afterglow In Gamma-Ray Bursts And Its Application

Gamma-ray Bursts (GRBs) mean a sudden release of γ — ray emission that last from milliseconds to thousands of seconds. With robust measurements of their redshifts, from 0.0085 to 9.4, and the typical isotropic luminosity is 1051 — 1052 erg/s, therefore GRBs are indeed the most luminous explosions since Big Bang. Since its discovery in 1967, GRBs have become one of the hot topic for astrophysical research. But the history of GRB research has been full of struggles, surprises, and excitements. After decades of observational and theoretical researches, one finally reaches a physical picture regarding the origin of GRBs. Even though many details are still unclear, but a general theoretical framework is set up, which is found successful in interpreting the multi-wavelength afterglows. The researches in my thesis focus on the GRB afterglows model and its application, including the standard blast wave model, circumburst medium, constrain the Lorentz factor of X-ray flares and the components of the outflows. The thesis is arranged as follows:In Chapter 1,I present a detailed review about GRBs, mainly on the progress of both ob-servational and theoretical studies. First I introduce the progress of GRB observational studies in different satellites eras, especially for the Swift and Fermi eras. Next, the internal shocks of prompt emissions and the external reverse-forward shock for very early afterglows are intro--duced. Finally, I present a brief introduction on progenitor, central engine, classification and some empirical correlations.In Chapter 2, we consider the early afterglows of Gamma-Ray bursts in a stratified medium with a power-law density distribution, and apply the early afterglows to determine the type of circumburst medium. A long-duration gamma-ray burst (GRB) has been widely thought to arise from the collapse of a massive star, and it has been suggested that its ambient medium is a homogenous interstellar medium (ISM) or a stellar wind. There are two shocks when an ultra-relativistic fireball that has been ejected during the prompt gamma-ray emission phase sweeps up the circumburst medium:a reverse shock that propagates into the fireball, and a forward shock that propagates into the ambient medium. In this paper, we investigate the temporal evolution of the dynamics and emission of these two shocks in an environment with a general density distribution of n ∝ R-k (where R is the radius) by considering thick-shell and thin-shell cases. A GRB afterglow with one smooth onset peak at early times is understood to result from such external shocks. Thus, we can determine the medium density distribution by fitting the onset peak appearing in the light curve of an early optical afterglow. We apply our model to 19 GRBs, and find that their k values are in the range of 0.4-1.4, with a typical value of κ～1, implying that this environment is neither a homogenous interstellar medium with κ= 0 nor a typical stellar wind with κ= 2. This shows that the progenitors of the their GRBs might have undergone a new mass-loss evolution.In Chapter 3, X-ray flares were discovered in the afterglow phase of gamma-ray bursts (GRBs) by the Swift satellite a decade ago and it is now known as a canonical component in GRB X-ray afterglows. we constrain the Lorentz factors of GRB X-ray flares using two different methods. For the first method, we estimate the lower limit on the bulk Lorentz factor with the flare duration and jet break time. In the second method, the upper limit on the Lorentz factor is derived by assuming that the X-ray flare jet has undergone saturated acceleration. We also re-estimate the initial Lorentz factor of GRBs, and find the coefficient is 1.67 rather than the commonly used 2 for interstellar medium (ISM) and 1.44 for wind case. We find that the correlation between the limited Lorentz factor and the isotropic radiation energy of X-ray flares in the ISM case is more consistent with that of prompt emission than the wind case’s in a statistical sense. However, the lower limit on Lorentz factor is statistically larger than the extrapolation from prompt bursts in the wind case. Our results indicate that X-ray flares and prompt bursts are produced by the same physical mechanism.In chapter 4, We work on a GRB sample whose initial Lorentz factors ((?)0) are constrained with the afterglow onset method and the jet opening angles (θj) are determined by the jet break time. We confirm the (?)0- Eγ,iso correlation by Liang et al. (2010), and the (?)0-Lγ,iso cor-relation by Lii et al. (2012). Furthermore, we find correlations between (?)0 and the beaming corrected γ-ray energy (Eγ) and mean γ-ray luminosity (Lγ). By also including the kinetic energy of the afterglow, we find rough correlations (with larger scatter) between (?)0 and the total (γ-ray plus kinetic) energy and the total mean luminosity, both for isotropic values and beaming corrected values:these correlations allow us to confront the data with GRB central engine models. Limiting our sample to the GRBs that likely have a black hole central engine, we compare the data with theoretical predictions of two types of jet launching mechanisms from BHs, i.e. the non-magnetized vv-annihilation mechanism, and the strongly magnetized Blandford-Znajek (BZ) mechanism. We find that the data are more consistent with the latter mechanism, and discuss the implications of our findings for GRB jet composition. In Chapter 5, we apply the theory of early afterglow about GRBs to predict the afterglows from fast radio bursts (FRBs), a new transients. The physical nature of FRBs is not identified. Detecting electromagnetic counterparts in other wavelengths is essential to measure their dis- tances and to settle down their physical nature. Assuming that at least some of them are of a cosmological origin, we calculate their afterglow lightcurves in multi-wavelengths (X-rays, optical and radio) by assuming a range of their total kinetic energies and redshifts. We focus on forward shock emission, but also consider the possibility that some of them might have bright reverse shock emission. In general, the FRB afterglows are too faint to be detected by curren- t detectors. Only if an FRB has a very low radiative efficiency in radio (hence, a very large kinetic energy), and when it is close enough, can its afterglow be detected in the optical and radio bands. We discuss observational strategies to detect these faint afterglows using future telescopes such as LSST and EVLA.Finally, I briefly present the urgent problems and outlook about GRBs.