Study On The Spectral Evolution And High Energy Afterglow Emission Of Fermi-LLE Gamma-ray Bursts

Most of the observed gamma-ray bursts spectra can be fitted by the Band function.The spectral parameters for the time-resolved spectra evolve with time.Besides,the radiation mechanism for high energy afterglow emission is still an open question.We will study the prompt spectral evolution of Fermi-LLE gamma-ray bursts and the high energy afterglow emission of GRB 180720B based on the observation data of the Fermi satellite in this thesis.We will extract the light curves by using gt Burst package and take RMFIT as the tool for making spectral analysis.GRB 180720B is one of the Fermi-LLE gamma-ray bursts.This burst is a very bright and long burst.After making the analysis,the spectral features in the prompt phase can be summarized as:（i）the“flux-tracking”pattern exhibited both for the low energy spectral indexαand the peak energy Ep in the Band function;（ii）four spectral parameter relations,E_P-F,α-F、E_P-α,andE_P-L_γ,sio,show strong positive correlations;（iii）the value of the low energy spectral indexαdoes not exceed the synchrotron limits for all time-resolved spectra;（iv）a single Band function is adequate to fit the joint GBM and LAT（include LLE）spectrum from 11 to 55 s after the GBM trigger.In the afterglow phase,six Ge V photons were detected when the X-ray flare was detected.The joint GBM and LAT spectrum can be fitted well by the Band function with the superposition of power-law at the moment.The results of the analysis suggest that synchrotron origin can account for the prompt emission of GRB 180720B and synchrotron self-Compton radiation can account for its high energy afterglow emission during the X-ray flare.After performing a detailed spectral analysis for the Fermi-LLE bursts,we found that（i）a single Band function is enough to perform the time-resolved spectral fitting for each burst around their peak flux;（ii）one can see that 77.8%of the bursts have an α_max,which is larger than the synchrotron limit;（iii）the typical value ofαis～-0.9 for the time-integrated spectrum,this may be due to the small number of gamma-ray bursts in our sample;（iv）75%of the bursts exhibit the“flux-tracking”pattern for the Ep evolution,and 77.8%of the bursts exhibit the“flux-tracking”pattern for theαevolution in our sample;（v）simulation shows that the two observed strong positive correlations,α-F and E_P-α,are artificial except for GRB 160905A in its α-F correlation.Besides,the results of the analysis suggest that the great majority of bursts in our sample are dominated by the synchrotron component in their prompt phase.