Statistical Properties And Origin Of The X-Ray Afterglow Plateaus Of Gamma-Ray Bursts

Gamma-ray bursts(GRBs)are erratic gamma-ray flashes from the sky,which are the most violent burst events in the universe after the Big Bang.In 1997,GRB afterglows were discovered.The redshifts of GRBs were determined,confirming their cosmological origin.After decades of observations and researches,people have established the basic picture of GRBs.The prompt emission comes from the internal energy dissipation(internal shock or magnetic reconnection process),while the multiband afterglow radiation comes from the interaction of the relativistic jet with the surrounding medium.Because of their extremely high luminosity,GRBs are expected to be observable at very high redshifts.Therefore,GRBs are considered to be a powerful tool for probing the early universe.However,the luminosity of GRBs covers several orders of magnitude compared to type Ia supernovae.Fortunately,some empirical correlations have been found between GRB parameters,either on the prompt emission or on the afterglow.Based on these empirical correlations,we may use GRBs as a standard candle to constrain the dark energy and cosmological parameters as in the case of SNe Ia.There is a special class of GRBs called "plateau" GRBs,which are characterized by a shallow decay phase followed by a normal decay phase in X-rays.The normal decay phase is consistent with the predictions of the external shock model,while the plateau phase is usually considered to be some additional energy injection into the external shock,possibly continuous injection from the central engine.Thus,observations of X-ray plateaus can help constrain various central engine models.In addition,some good correlations have been found in plateau GRBs.Xu & Huang(2012)found an L-T-E three-parameter relationship between the isotropic prompt radiation energy,end time of the plateau,and the plateau luminosity.In this thesis,we focus on these plateau GRBs.Based on the Markov chain Monte Carlo method,we constrained the magnetar model of GRBs by fitting the L-T-E threeparameter correlation.We calculate the magnetic field strength and the initial period of magnetar by the end time of the plateau,and the plateau luminosity.We discuss the GRB central engine models and some possible energy extraction mechanisms.The contents of this thesis are as follows.Chapter 1 is a review of the current state of researches on GRBs,introducing the basic picture of GRBs,the observations and the theories of afterglow,and the central engine models.In Chapter 2,we collect 174 plateau GRBs observed by Swift,and use this expanded sample to statistically study the L-T-E three-parameter relation.The results further confirm the existence of the L-T-E three-parameter relation and can be used as a support for the magnetar energy injection model.In Chapter 3,we use the magnetic dipole radiation model to calculate the magnetic field and initial period of the central magnetar for plateau GRBs.We found that there may be a correlation between the magnetic field strength and the initial period of magnetars,and we discuss the implications of this correlation for the central engine model.We introduce the energy injection efficiency and study the variation of the magnetar parameters under various energy injection efficiency.In Chapter 4,we give some discussions and an outlook.