The Study Of The Early And Very-High-Energy Afterglow Radiation Physics Of Gamma-ray Bursts

The very early（<1hr）and very high energy（VHE,>100Ge V）afterglows of gamma-ray bursts（GRBs）carry crucial information about the properties of GRB jets,particle acceleration,and radiation mechanisms.Observational research on the early afterglows of bursts has made significant progress since the launch of the Swift satellite in 2004,and rich data has been accumulated.The VHE afterglows of GRB 190114C were observed with the MAGIC telescope at a confidence level of＞50σon January 14,2019.This formally opens the era of VHE afterglow observations of GRBs.This thesis reviews the observational and theoretical progress on GRBs and their afterglows,and presents the results of our observational and theoretical investigations on the very early and VHE afterglows of GRBs based on the standard external shock model for deepening the understanding of some fundamental issues,such as the physical properties of GRB jets,ambient environment,particle acceleration,and radiation mechanisms.In the standard afterglow theoretical framework,the external shock model successfully explains the phenomenon of GRB afterglows.The long-lasting forward shock is formed when it propagates through the surrounding medium.By collecting observational data of optical and X-ray afterglows since the launch of the Swift satellite,we obtained a sample of 30 GRBs.Basing on the standard forward shock model,we fitted the optical and X-ray afterglow lightcurves of these GRBs and provided the posterior distributions of the model parameters by employing the Bayesian inference methods.We also calculated related physical quantities of the jets.It is found that the GRB radiation efficiency distribution of this sample ranges from 0.04%to 10%,being consistent with the low radiation efficiency as predicted by the matter-dominated jet model.The distributions of both the geometrically-corrected prompt radiation energy and jet kinetic energy are normal distribution in narrow ranges,which is consistent with the argument of a standard energy reservoir for GRB jets by previous studies.Particularly,a tight relationship among the deceleration radius(R_dec),initial magnetic field strength（B₀）,and the energy ratio of magnetic field to electrons（σ_B=ε_B/ε_e）is found,i.e.R_dec,17^r=10^1.00±0.05B₀^-0.83±0.06σ_B^0.26±0.02,which inferrs a quasi-universal electron energy among these GRBs.The shock propagates back into the ejecta,generating short-lived reverse external shocks.In 2021,the GWAC-F60A telescope observed a bright early optical flash of GRB 210104A.Spectroscopic observations with a 2.16-meter optical telescope was made in the Target of Opportunity（To O）model,and the redshift of the burst is measured as 0.46.By fitting its optical and X-ray afterglow light curves using the forward and reverse shock models,it is found that its early optical flash is dominated by the reverse shock radiations,and the circum-burst medium is extremely dense(n₀～416.9 cm^-3).Its jet has a low kinetic energy(E_k,iso=4.5×10⁵¹erg),a moderate initial Lorentz factor（Γ₀～35）,a moderately magnetized factor（with a ratio of reverse to forward shock magnetization factorR_B=2 8）,and a high radiation efficiency（η_γ=77%）.In addition to the research on early afterglows,the observation and study of VHE afterglows of GRBs are also crucial for revealing the characteristics of GRB jets and radiation physics.This thesis systematically investigates three events with VHE afterglow detection,i.e.GRBs 190829A,201015A,and201216C for revealing their prompt and afterglow radiation physics.Our data analysis indicates that the circumburst medium of the nearby low-luminosity GRB 190829A（z=0.0785）is extremely dense and strong optical extinction is found（A_V=2.33mag）.We proposed model to self-consistently explain the observed characteristics of both prompt and afterglow emission.In our model,it was suggested that very early high-energy gamma-ray photons during the prompt radiation phase are scattered by the dense medium.This process results in a bright succeeding X-ray pulses and an electron-positron-pair-riched shell through the annihilation of these high-energy photons.Bright optical and X-ray humps are observed when the jet passes through the shell.The model assumes that multi-band afterglow radiations are produced by synchrotron radiation and self-Compton scattering（SSC）of relativistic electrons in the jet.The model not only successfully fits the multi-band afterglow light curves of GRB 190829A,but also resonabally predicts the radiation of VHE gamma-ray afterglows in the Te V energy range,which is in good agreement with the observational data published by the H.E.S.S.collaboration later.Data analysis of the prompt radiation characteristics of GRBs 201015A and 201216C shows that GRB201015A is an extremely soft case（E_p=14 keV）,while GRB 201216C is a relatively hard event（E_p=438 keV）.GRB 201216C is currently the highest-redshift VHE gamma-ray burst event detected（z=1.1）.By fitting their radio,optical and X-ray afterglow light curves,we inferred their VHE radiation flux based on the SSC scattering mechanism.The predicted VHE afterglow fluxes are almost consistent with the observational data reported by the MAGIC collaboration later.Our analysis for the three GRBs indicates that their VHE afterglow radiation can be explained by the SSC scattering mechanism of electrons accelerated by forward shock.Through the analysis of current GRBs with detections of VHE gamma-ray afterglows,it is found that the parameter distributions of these events is broad,i.e.isotropic total energy distributed in E_γ,iso=10⁵⁰～6×10⁵³erg,medium density distributed inn₀=2～1202 cm^-3,peak energy distribution of prompt gamma radiation withinE_p=14～620 keV,and initial Lorentz factor distribution of jets withinΓ₀=30～331.These results indicate that GRBs with VHE afterglow detection do not exhibit common characteristics.The results presented in this thesis suggest that observations of both early and VHE afterglows of GRBs can be explained using the standard external shock model.Furthermore,we have found that these bursts also satisfy the previous three-parameter relation ofL_γ,p,iso-E_p,z-Γ₀.The observational windows for VHE gamma-rays and multi-messenger of GRBs have been opened.The next-generation facilities,such as the Cherenkov Telescope Array（CTA）,the Space Variable Object Monitor（SVOM）,and the Einstein Probe（EP）,which are operational currently or in the near future,will provide new opportunities for refreshing our understanding on the physical characteristics of GRB jets,medium environments,particle acceleration,and radiation mechanisms.