Study On The Nature Of Grb Prompt Emission And Their Circumburst Environment Within The External Shock Models

Gamma-ray burst(GRB)are originated from the death of massive stars or mergers of compact binaries.The most popular model for GRBs is the standard fireball model,in which the broad-band afterglow is from external shock,while the prompt gamma ray emission is from internal shocks.However,observationally there do exist some GRBs that only have a single smooth peak,or just a few long and temporally separated peaks with a large single smooth peak.The simple smooth profiles could arise from an external shock.GRB 120729A is a single smooth pulse of gamma ray emission,which extends continuously to the X-rays.It exhibits achromatic light-curve behavior,consistent with the predictions of the external shock model.The light curves are decomposed into four typical phases:onset bump,normal decay,shallow decay and post-jet break.The spectral energy distribution(SED)evolves from prompt y-ray emission to the afterglow with photon index from 1.36 to 1.75.There is no obvious evolution of the SED during the afterglow.The multiwavelength light curves from y-ray to optical can be well modeled with an external shock by considering energy injection,and a time-dependent microphysics model with?B/taBfor the emission at early times.Therefore,we conclude that both the prompt y-ray emission and afterglow of GRB 120729A have the same external shock physical origin.The parameter are EK,iso=3.36×1054erg??O=760?n=8 cm-3??e=0·01?#B=[9×10-5,4.0×10-6]??j=0.02rad?LO=5.0×1050 erg s-1?q=-0.1?ts=0 s and te=3698 s.Our model indicated that the ?B evolution can be described as a broken power-law function.We also systematically investigate single-pulse GRBs in the Swift era.The criterion for its selection are single pulses,smooth connection between the BAT and XRT data with a slope less than the typical value of 1.5,have early optical data.It is finding that only a small fraction of GRB 051111 and 070318 are likely to originate from an external shock for both the prompt y-ray emission and afterglow.We further discussed the prompt emission nature of the GRB 120729A,051111 and 070318.If the composition of the GRB jet is a matter-dominated fireball,the absence of a "prompt emissionl" signal caused by internal dissipation within the jet is puzzling.It suggest that the outflow is Poynting-flux dominated.Keep a moderate ? not only suppress the photospheric luminosity,but also suppressed the reverse shock.This is consistent with the composition of GRB 120729A,051111 and 070318 that have not been observed in optics for the reverse shock wave.