Study On The Jet Dynamics And Radiation Physics Of Gamma-Ray Bursts

Gamma-ray bursts(GRBs)are an extremely high-energy astronomical explosion phenomenon.The basic physical picture is that extreme relativistic jets driven by the collapse of massive stars or merges of binary compact stars,and internal dissipation or the photosphere of jets produces the prompt emission of GRB.External shock is formed when the jets sweep the circum-burst medium,producing the external shock afterglow.This thesis aims to study the jet dynamics and the radiation physics of GRBs.In Section 1,we mainly introduce the basic physics of GRB,the observed characteristics of the prompt emission and afterglow,the jet dynamics and radiation physics,the main problems in GRB physics,and the motivations of this thesis.GRB jet dynamics is a key to understand the radiation physics of GRB.Previous study focused only on the Lorentz factor at specific phases in GRB process,however,the evolution of the jet Lorentz factor is crucial to understand the GRB radiation physics.Theoretically,the value of the Lorentz factor of jets can be calculated by high-energy spectral cutoff in the GRBs’prompt emission owing to the absorption of photon–photon pair productions.In Section 2 we search for GRBs that have at least two pulses with high-energy spectral cutoff in their radiation spectrum from the Fermi satellite data.Then,we fit the pulse light curve and energy spectra to calculate the Lorentz factor of the jets.We find nine special GRBs and it is found that:in most of the GRBs,the Lorentz factor of jets generally fluctuates near a certain range and without a specific trend;the Lorentz factor of jets seems to increase with time in GRB 130821A,GRB160509A and GRB 160625B.For these nine special GRBs,we also examine the relations among the isotropic gamma-ray luminosity,the cosmological rest-frame peak energy,and the Lorentz factor for pulses in sample.We show that these relations are consistent with the results of previous studies on large sample bursts.The important information related to the explosion process and very early phase of jet dynamics might exist in very early prompt emission of GRBs.In Section 3,we investigate the origin of the very early prompt emission of GRB200829A.The energy spectrum analysis for the main pulse,exhibits a two-component radiation spectrum,which can be fitted with the Band function by combine a blackbody radiation spectrum,especially near the peak.Based on the blackbody component,we investigate the photospheric properties and find that the photospheric is moderately magnetized when the initial radius of the fireball is taken as 10~9cm.This implies that the Band component of the pulse may originate from the dissipation of magnetic energy.The energy spectrum analysis of the very early prompt emission exhibits that it can be fitted by a power-law function with an index of(25)-1.7,which is consistent with the spectral index of the late-time X-ray afterglow energy spectrum.Therefore,we propose that the very early prompt emission of this GRB may originate from the external shock.The main gamma-ray pulse came from the re-activity of the central engine,and forms a X-ray bump after the main pulse by energy injection.With a MCMC fitting method,it is found that the very early prompt emission,X-ray bump,and subsequent afterglow of GRB 200829A are well described in this scenario.In Section 4,we perform a detailed analysis of the observational data of GRB 140419A to study its radiation physics and jet properties.Observationally,the prompt emission light curve of GRB 140419A featured many pulses overlapping;the early optical afterglow phase decays with an index of(25)-1.2and is followed by a peculiar bump in the late phase;and X-rays show a plateau at(25)1000s after the burst trigger.For the achromatic behavior of the X-rays and optical afterglow,we carry out a joint energy spectrum analysis of XRT and BAT or optical,and find that the X-ray light curve and energy spectrum index satisfy with the closure relation of slow cooling phase of the afterglow radiation,while the X-ray plateau satisfy without this relation.We argue that the X-ray plateau is contributed by the magnetic dipole radiation,while the optical afterglow is contributed by the external shock radiation.Then we constrain the parameters associated with the magnetar as the central engine of GRB 140419A.Based on the MCMC fitting method,the observational data can be well described in the above model.In Section 5,we summarize the above work and finally make a prospect for the future study.