Study On The Variability And Energy Spectrum Of Photospheric Emission In Gamma-ray Burst

This thesis is dedicated to studying the photospheric emission of jets in gamma-ray bursts（GRBs）.The variability and radiation energy spectrum of photospheric emission are our main focuses.Firstly,the relations between the variability of the photospheric emission and the variability of the jet dynamic properties are explored.We also discuss the dynamic of the jet based on the above relations.Secondly,we propose an analytic radiation spectrum to describe the theoretical radiation spectrum of the photospheric emission obtained by numerical simulations.The photospheric emission is closely related to the dynamic parameters,e.g.,jet power,dimensionless entropy and so on.Then,the dynamic of the jet may be estimated by studying the variability of the photospheric emission.We find that the variability of the photospheric emission strongly depends on the fluctuations of the power and dimensionless entropy of the jet.By setting a constant dimensionless entropy of the jet,we find that the light curve of the photospheric emission shows a“tracking”pattern on the time profile of jet power.However,the relative variability is significantly low in the photospheric emission compared with that in the jet power.If the dimensionless entropy of the jet is a genetic variable,the“tracking”pattern of photospheric emission on the fluctuations of jet power would be lost.The variability of photospheric emission would be complex.In addition,the variability of the photosphericemission is energy band-dependent.In some certain energy bands,the variability of the photospheric emission becomes obviously weak.The variability of the photospheric emission on the low energy band may have an opposite phase to the total energy band.The thermal components of some GRBs have been detected are discussed.In general,the blackbody（BB）spectrum or multicolor-blackbody spectrum with a power-law distribution of the thermal temperature（PL-m BB）is used to fit the photospheric emission observed in gamma-ray bursts.However,we point out that two kinds of radiation spectrum could not well describe the photospheric emission.Based on the numerical results about the photospheric emission spectra,we suggest that the multicolor-blackbody spectra with a cutoff power-law distribution of the thermal temperature（CPL-m BB）rather than BB or PL-m BB are more approximately to describe the photospheric emission.By fitting the observations with CPL-m BB,we find that CPL-m BB can well describe the observed thermal components in GRBs.The information on the photosphere radius and the bulk Lorentz factor can be deduced from the fitted observations.Moreover,the analysis of L_ph-E_p relationship show that∝P_jet^0.34 should be satisfied to match the statistical relation E_p∝L^0.5±0.1.