Constraints On High Energy Afterglow Emission Properties Of Gamma-Ray Bursts

It is proposed that the afterglows of gamma-ray bursts(GRBs) are produced by the external shocks due to the ejecta-medium interaction.The multi-wavelength GRB afterglows,especially the high-energy afterglows,are probes for ambient medium and the mechanism for particle acceleration.The multi-wavelength observation of GRB190114C strongly supports that the sub-Te V photons are dominated by the Synchrotron selt-Compton(SSC)process for the first time.This thesis first briefly summaries the progress of GRBs and their afterglows in recent years,and gives a comprehensive introduction to the GRB afterglow theory,then presents our analysis results on study on properties of the high energy GRB afterglows.We first present a comparative analysis on the SSC emission of GRB afterglows in the homogeneous and wind environment in the framework of the forward shock mod-el.The ?? absorption of very high-energy photons due to pair production within the source and the Klein-Nishina effect on the inverse-Compton scattering are considered.Generally a higher SSC flux is expected for a larger circum-burst density due to a larg-er Compton parameter,but meanwhile the internal ?? absorption is more severer for sub-TeV emission.The flux ratio between the SSC component and the synchrotron component decreases more quickly with time in the wind medium case than that in the homogenous-density medium case.The light curves of the SSC emission are also different for the two types of media.We also calculate the cascade emission resulted from the absorbed high-energy photons.In the ISM environment with n(?)1 cm^-3,the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band,which may flatten the optical afterglow light curve at early time(t<1 h).In the wind medium with A_*(?)0.1,the cascade emis-sion in the e V-Ge V band is comparable or even larger than the emission of the primary electrons at early time.Bimodal spectral energy distributions of gamma-ray bursts afterglow of GRB-s 190114C,130427A and 180720B confirm that they are originated from the syn-chrotron emission and synchrotron self-Compton Scattering process of electrons accel-erated in the jets.The radiation mechanism and the physics of the observed spectrum-luminosity/energy relations of GRBs remain as open questions.By extracting the Syn component through fitting their early afterglow SEDs with the Syn+SSC model,we find that their luminosity(L_syn),peak energy(E_p,syn,z),and the Lorentz factor of the afterglow fireball(?_t) follow the L_p,iso-E_p,z-?₀ relation of prompt gamma-rays,where L_p,iso is the isotropic luminosity,E_p,zis the peak energy of the?f_?spectrum in the burst frame,and ?₀ is the initial Lorentz factor of the fireball.To examine whether late afterglows is consistent with this relation,we calculate the synchrotron compo-nent at late afterglows.It is found that they also follow the same L_p,iso-E_p,z-?₀ relation,albeit they are not consistent with the L_p,iso-E_p,zrelation.Our results may imply that the L_p,iso-E_p,z-?₀ would be an universal feature of synchrotron radia-tions of electrons accelerated in GRB jets throughout the prompt and afterglow phases among GRBs.Its origin is not fully understood and possible explanations are briefly discussed.High energy photon radiations of gamma-ray bursts and active galactic nuclei are dominated by their jet radiations.We examine whether the synchrotron radiations of jets in BL Lacs,flat spectrum radio quasars(FSRQs),and Narrow Line Seyfert 1galaxies(NLS1s)follow the relation between the prompt gamma-ray emission and the initial Lorentz factor(?₀) of GRBs.It is showed that the AGN sample does not agree with the L_p-E_p,z-?₀ relation of GRBs.In addition,we obtain a tight relation of L_syn?E⁰_sy._n⁴⁵,_p^±0.15?^3.50±0.25 for FSRQs and NLS1 galaxies,where L_synis the luminosity at peak photon energy E_syn,pof the synchrotron radiations.This relation is different from the L_p-E_p,z-?₀ relation of GRBs.The dependence of L_synto?is consistent with the expectation of the Doppler boosting effect for the FSRQs and NLS1 galaxies,but it is not for GBRs.We argue that?₀ may be a representative of the kinetic power of the radiating region and the tight L_p-E_p,z-?₀ relation is shaped by the radiation physics and the jet power together.In summary,by analysing several multi-wavelength data for several GRBs with detection of sub-Te V afterglow emission,we found that the GRB very-high-energy afterglows show diverse phenomenology in different circum-burst mediums.A higher SSC flux is expected for a larger medium density.On the other hand,the internal??absorption is more severer for sub-Te V emission in a denser medium environment.The cascade emission resulted from the absorbed high-energy photons could be comparable to the synchrotron emission of the primary electrons in the optical band,which may be an interpretation of the optical and X-ray afterglow lightcurves at early time.In addition,by examining the observed GRBs 190114C,180720B and,130427A,we find that their prompt emission and synchrotron components of afterglow follow the L_p,iso-E_p,z-?₀ relation of prompt gamma-rays.This relation may be a hint for feature of synchrotron radiations of electrons accelerated in GRB jets throughout the prompt and afterglow phases among GRBs.Furthermore,we take a comparative studies of the similarity between the jet radiations from GRBs and AGNs,and explore the universal physics of radiation physics.The very-high-energy window in multi-messenger era is opened for GRB study.With unprecedented high sensitivity in the GeV-TeV band of the Cerenkov Telescope Array(CTA) and Large High Altitude Air Shower Observatory(LHAASO),we will have great opportunity for revealing the accelerated electrons and radiation mechanism of GRB jets in the near future.