Spectral Evolution And Cosmological Properties Of Gamma-Ray Bursts

We summarize the observational and theoretical progress on gamma-ray bursts (GRBs) and their afterglows, then present our analysis results on the spectral evolution and cosmological properties of GRBs in this thesis.Thanks to the broadband observations for GRBs with the Fermi mission, we made a time-resolved spectral analysis for51long and11short bright Femri/GBM GRBs to study the evolution of Ep within an individual GRB. Among8single-pulse long GRBs,5show hard-to-soft evolution, and the other3show intensity-tracking. The multi-pulse long GRBs have more complicated patterns. Among the GRBs whose time-resolved spectrum is available for the first pulse, almost half bursts show clear hard-to-soft evolution, and the other half bursts show clear intensity-tracking. Later pulses typically show the tracking behavior, although a hard-to-soft evolution pattern was identified in the second pulse of2GRBs whose pulses are well separated. Statistically, the hard-to-soft evolution pulses tend to be more asymmetric than the intensity-tracking ones, with a steeper rising wing than the falling wing. Short GRBs have Ep tracking intensity exclusively with the16ms time resolution analysis. We performed a simulation analysis, and suggested that at least for some bursts, the late intensity-tracking pulses could be a consequence of overlapping hard-to-soft pulses. However, the fact that the intensity-tracking pattern exists in the first pulse of multi-pulse long GRBs and some single-pulse GRBs suggest that intensity tracking is an independent component. For the GRBs with measured redshifts, we present a time-resolved Ep-Lγ correlation analysis and show that both the slope and the scatter of the correlation is comparable to that of the global Amati/Yonetoku relation. We discuss the predictions of various radiation models regarding Ep evolution, as well as the possibility of a precession jet in GRBs. It seems that the data pose great challenge to all these models, and hold the key to unveil the physics of GRB prompt emission.An apparent redshift z dependence of the jet opening angles θj of GRBs is observed. The jet opening angles for high-z bursts should be much smaller than for low-z bursts. The evolution feature of the GRB jet opening angle is based on the statistics for the current observational samples. However, it well known that observational samples are widely suffered from instrumental selection effects. We investigate whether this dependence can be explained with instrumental selection effects by a bootstrapping method. We consider various instrumental selection effects, including the flux threshold and the trigger probability of BAT, the probabilities of a GRB jet pointing to the instrument solid angle, and the probability of redshift measurement. Our results can well reproduce the observed θj-z dependence and indicate that the dependence can be explained with instrumental selection effects.As detection number of high-z GRBs (z>4) increases, it is found that the observed high-z GRB rate shows significant excess over that estimated from the star formation history. We also investigate the possible physical reasons to explain the excess based on Monte Carlo simulations, including the cosmic evolution of GRB luminosity function.