Constraints On The Nature Of GRB Jet And Circumburst Environment Within The Framework Of Forward Shock Models

During the graduate period,my work is mainly reporting our optical observations of GRBs and investigating the composition of jets and the circumburst environment.The detail is constraining the model parameters by reproducing the afterglow light curves within the framework of standard forward shock models,based on the Markov Chain Monte Carlo algorithm,then acquiring the parameter distributions and their possible correlations,and finally discussing the nature of GRBs.The results are as follows:For GRB 110530A,its R band light curve shapes a peculiar broad bump followed by a delayed plateau,which is consistent with what X-ray afterglow and other optical bands behave.The optical and X-ray afterglows are well reproduced within the framework of standard forward shock models when an extra delayed energy injection is added.Our modeling analysis shows the composition of jet should be matter-dominated in prompt emission phase as well as afterglow phase due to the extremely low radiative efficiency and magnetization parameter.And the delayed energy injection may stem from the fallback accretion of its pre-supernova.For GRB 140629A,its optical light curve can be very well interpreted with the standard forward shock models.The jet should be also matter-dominated as its radiative efficiency is very small.In addition,it well agrees with the Lp,iso-E'p-? relation.For modeling 24 optical and X-ray afterglows with the standard forward shock models,we obtain the most possible parameter distributions for the shock energy to magnetic field ?B,the shock energy to electron ?e?,the isotropic kinetic energy EK,iso,the initial Lorentz factor ?0,the medium density n,the electron distribution index p,the radiative efficiency ??,the magnetic field strength B and the magnetization parameter ?.Based on the analysis of parameter correlations,it is found that both the magnetic field strength and the magnetization parameter have a relatively weak positive connection with the radiative efficiency,to some extent,which suggests that the magnetic property of composition in afterglow phase would be positively connected with that in prompt emission phase,no matter what complicated processes have ever happened between these two phases.More importantly,the majority of GRBs should be matter-dominated in both prompt emission phase and afterglow phase according to the results of radiative efficiency and magnetization parameter.