Broadband Emission From GRB Afterglows And Gamma-ray Pulsar Binaries

Gamma-Ray Bursts (GRBs) are short and intense bursts of gamma-rays from universe. They are the most violent explosions after the Big Bang. Since the accidental discovery in1967(reported in1973), the origin and the corresponding physical process in GRBs have always been one of the most interesting topic in astrophysics. After the discovery of afterglows, which are the long lasting counterparts of GRBs in the low energy bands, in1997, the GRB research enters a golden era. 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 standard model of GRBs, but also help us to understand the microphysical processes and the radiation mechanism in the shock, the structure of the surrounding environment, and the characteristics of the host galaxy.In addition to GRBs on the cosmological scales, some pulsar binary systems in our Galaxy can also produce gamma-rays and multi-band radiation. Such systems are composed of pulsars and young massive stars, and PSR B1259-63/LS2883is the only confirmed one at present. In addition, although the types of the compact objects in LS5039, LS I+61°303, HESS J0632+057and1FGL J1018.6-5856are still unclear, it is generally believed that there are pulsars in these systems. The gamma-ray pulsar binaries usually emit multi-band radiation, i.e. from radio to TeV gamma-rays, and the light curves are modulated on the orbital periods. The research on the multiband radiation by using the wind interaction model can help us understand the nature of the pulsar wind and the stellar wind.In this thesis, the broadband emission from GRB afterglows and gamma-ray pulsar binaries is investi-gated. We model the multiband GRB afterglows to test the generic dynamical model of GRB afterglows, reproduce the rebrightenings in GRB afterglows, and model the multiwavelength light curves of PSR B1259-63/LS2883. The structure of the thesis is as follow:In Chapter1, we introduce the progress of the GRB studies. We first review the observations of the GRBs and their afterglows in the past40years, including the new results observed by the Swift and Fermi satellites. The standard fireball model and the standard afterglow model are also investigated in detail. Finally, we present a brief introduction on the central engines and the GRB cosmology.A brief review on the gamma-ray pulsar binary systems is given in Chapter2. We first investigate the multi-band observational characteristics of PSR B1259-63/LS2883, LS5039, LS I+61°303, HESS J0632+057and1FGL J1018.6-5856in detail, and then introduce the wind interaction model of the gamma-ray pulsar binary system, including the geometry of the bow shock, the magnetic field and the electron distribution in the shock, the radiation processes and the pair production in the binary systems. In Chapter3, we test the generic dynamical model of GRB afterglows by modelling the multi-band afterglow light curves of GRB980703. This GRB has extensive broadband afterglow data. More im-portantly, its radio afterglow was very bright and monitored until more than1000days after the burst. Additionally, there is no obvious special features, i.e., no rebrightenings, no plateau, and no special steep decay or slow decay in the multi-band afterglow light curves. All these conditions make GRB980703be one of the most valuable samples in GRB researches, especially in investigating the behavior of af-terglows in the deep Newtonian phase. We use a generic dynamic model, which has the virtue of being applicable for both the radiative and adiabatic cases and in both the ultrarelativistic and nonrelativistic phases, to fit the broadband afterglow of GRB980703. Our results show that the model can give a satisfactory explanation to the multi-band and overall afterglow light curves. We also obtain some phys-ical parameters on the GRB outflow and the surrounding environment, and the host galaxy extinction is estimated.In Chapter4, we propose a possible explanation for the rebrightenings in GRB afterglows. With the advance of observation techniques and the accumulation of observational data, especially after the launch of Swift satellite, a lot of unexpected behaviours appear in GRB afterglows. One of them is the rebrightenings in many GRB afterglow light curves. We find that adding the wind bubble environment structure and the variation of the shock microphysics parameters in the generic dynamical model of GRB afterglows can well explain the rebrightenings in GRB afterglows. We investigate the effects of various parameters on the afterglows, and reproduce the R-band and X-ray afterglow light curves of GRB060206, GRB070311and GRB071010A.In Chapter5, we reproduce the multiwavelength light curves of PSR B1259-63/LS2883, which is the only gamma-ray binary whose compact object is confirmed as a pulsar. The X-ray and TeV light curves of this system are similar and display two-peak profiles. Recently, the Fermi observations show that there is no significant signal in the GeV light curve before periastron, but there is an interesting flare after periastron. These multi-band characteristics are difficult to explain in the standard wind interaction model. Some hydrodynamic simulations show that in the tail of the bow shock produced by the wind interaction, the bulk motion of the particles can reach the mild-relativistic or ultrarelativistic velocities. We find that the Doppler boosted synchrotron radiation could well reproduce the observed GeV flare. We also find that the anisotropy in the pulsar wind, which has been suggested in the research of Crab Nebula, plays a significant role in producing the two-peak profiles in both X-ray and TeV light curves. We add the above two effects in the wind interaction model to fit the multi-band light curves and the spectra of PSR B1259-63/LS2883satisfactorily.Finally, we give some discussions and an outlook in Chapter6.