| Gamma-Ray Bursts(GRBs)are short and intense flashes of gamma-rays observed from the sky in arbitrary directions.They are one of the most violent stellar explosions after the Big Bang.After the discovery of afterglows in 1997,which are the long last-ing counterparts of GRBs in the low energy bands,the GRB research enters a golden era.Thanks to the prior satellites(e.g.,BeppoSAX,Swift,Fermi),people now un-derstand that at least some long GRBs come from the collapse of massive stars after researches of nearly half a century.Recently,the association between GRB 170817A and GW 170817 indicates that at least part of short GRBs are due to the neutron s-tar-neutron star mergers.At present the multiband emission spanning from radio to GeV gamma-rays of GRB afterglows has been widely observed.The research on the multi-band GRB afterglows can not only test and improve the model of GRBs,but also help us to understand the microphysical processes and the radiation mechanism in the shock,the structure of the surrounding environments,and the characteristics of the host galaxies.In this thesis,we model the multiband GRB afterglows to test the generic dynamical model of GRB afterglows and reproduce the rebrightenings in some peculiar afterglows.The structure of this thesis is organized as follows.In Chapter 1,we briefly introduce observations and theories on the GRB prompt emission and the afterglows.Our researches in GRB field are presented in Chapter 2.We use a numerical method to calculate the dynamics of outflow and reproduce the peculiar GRB afterglows based on the forward shock model.In our study,we con-sider constant energy injection,the delayed energy injection caused by the fall-back accretion of black holes and the injection of the magnetic dipole radiation from the central magnetar.In chapter 3,by invoking an asteroid disrupted and accreted by P-SR J0738-4042,we show that the pronounced change in the spin-down rate can be reasonably explained.In particular,the sudden initial reduction of the spin-down rate comes from the initial accretion when the asteroid flies by the central pulsar for the first time,while the subsequent drop is due to the bound gaseous debris flying away.The relatively stable spin-down rate at the last stage can be explained as the result of con-stant accretion from the transient accretion disk formed after the orbital circularization.Recently,GRB 170817A was confirmed to be associated with GW 170817,which was produced by a neutron star-neutron star merger.Theoretically,it is widely accepted that short gamma-ray bursts can be produced by two distinctly different mechanisms,binary neutron star mergers and neutron star-black hole mergers.Motivated by this idea,in chapter 4,we collect a universal data set constituted of 51 short gamma-ray bursts observed by Swift/BAT,among which 14 events have extended emission com-ponent.We study the observational features of these 51 events statistically.It is found that our samples are consisted of two distinct groups.They clearly show a bimodal distribution when their peak photon fluxes at 15-150 keV band are plotted against the corresponding fluences.We argue that those short gamma-ray bursts with extended e-mission come from the coalescence of neutron stars,while the short gamma-ray bursts without extended emission come from neutron star-black hole mergers.In the last chapter,we give a summary of the thesis,and also discuss some poten-tial studies that could be done in the future. |
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