Pulse Temporal Characteristics And Their Energy Dependence Of Gamma-Ray Bursts

Gamma-ray bursts(GRBs)are the most luminous events observed so far. Their light curves are complicated with many overlapping pulses.The pulses are the "building blocks" of the light curves,which may represent the fundamental physical process of this phenomenon.I investigate the temporal characteristics of the pulses and explore their possible implications to the GRB physics.This thesis summarizes the progress of our understanding the nature of this phenomenon from both observations and theory,and then presents our research work in detail.It is generally suggested that long-lag,wide-pulse GRBs are subluminous. Whether they form a peculiar subclass of GRBs is still a puzzle.Their simple temporal structures might have simpler physics.We study the pulse temporal characteristics and their energy dependence of these bursts.Our main results are as following:(1)We find that the pulses tend to be narrower and more symmetry at higher energies,and the pulse peak time,width,rise time scale and decay time scale are power-law functions of energy,but the distributions of the power-law indices in different bursts have large dispersions.This implies that the energy dependence of the temporal characteristics may not be the same for different bursts.(2)We find that the pulse peaks shift to later times at lower energies,and their light curves show significant spectral lags.We measure the three types of pulse spectral lags:peak lag(τ_peak),cross correlation function (CCF)lag(τ_CCF)and centroid lag(τ_cen),and find that the three quantities are correlated,butτ_cenis systematically larger than bothτ_peakand TOTE,andτ_CCF and T_peakare highly correlated,while the other pairs of variables are less well correlated.These indicate that the different types of lag measurements could represent the different aspects of pulse evolution,withτ_CCFrepresenting the shifting of pulse peaks andτ_cendescribing the stretching of pulses.In addition, we also find that the pulse spectral lags relative to the same low-energy band increase with the energy of the corresponding high-energy band,but the energy dependence of the lags is shallower at higher energy bands.(3)Besides the width - energy relation was found previously,the relationships between other quantities are studied for the first time.Based on these results and some previous investigations performed by other authors,we tend to believe that all energydependent pulse temporal properties may come from the joint contribution of both the hydrodynamic process of the outflow and the curvature effect,where the energy-dependent spectral lag may be mainly dominated by the dynamic process,and the energy-dependent pulse width may be mainly determined by the curvature effect.(4)GRB 980425 and GRB 060218 are two prototypes of low-redshift,low-luminosity GRBs.We find that the temporal characteristics of the two bursts are well consistent with those of other long-lag,wide-pulse GRBs. This suggests that the two bursts may share the similar radiation physics with normal long-lag,wide-pulse GRBs.The relationship between pulse width and energy has been studied extensively, with the power-law index being about -0.4.It is very interesting whether this correlation can be extended to X-ray bands.The prompt emission of GRB 060124 from X-ray to gamma-ray bands are simultaneously observed by Swift, and its pulses can be well identified over a broad energy bands.Take GRB 060124 as an example,we present a detailed analysis on the temporal properties of the two well identified pulses of this burst from X-ray to gamma-ray bands.We find that the pulses are narrower at higher energies,and both X-rays and gamma-rays follow the same width - energy relation for an individual pulse.We also find that the rise-to-decay ratio seems not to evolve with energy and the ratio values are well consistent with that observed in typical GRBs.These results indicate that the X-ray emission is consistent with the spectral behavior of the gamma-ray emission and the emissions in the two energy bands are likely to be originated from the same physical mechanism.