Early afterglows from radially structured outflows and the application to X-ray shallow decays  

作　　者：Xue-Wen Liu Xue-Feng Wu Yuan-Chuan Zou Tan Lu 

机构地区：[1]Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China [2]Graduate University of Chinese Academy of Sciences, Beijing 100049, China [3]Joint Center for Particle Nuclear Physics and Cosmology （J-CPNPC）, Nanjing 210093, China [4]Department of Astronomy ＆ Astrophysics, Pennsylvania State University, University Park, PA 16802, USA [5]Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, China

出　　处：《Research in Astronomy and Astrophysics》2009年第8期911-920,共10页天文和天体物理学研究（英文版）

基　　金：supported by the National Natural Science Foundation of China(grant Nos. 10473023, 10503012, 10621303, 10633040 and 10703002);the National Basic Research Program of China (973 Program 2009CB824800);XFW also thanks the support of the NSF AST0307376, NASA NNX07AJ62G, NNX08AL40G, the China Postdoctoral Science Foundation, and the Postdoctoral Research Award of Jiangsu Province

摘　　要：In the fireball model, it is more physically realistic that ganuna-ray burst （GRB） ejecta have a range of bulk Lorentz factors （assuming M ∝ Г^-8）. The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases： the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sad ＆ Meszaros （2000）. However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.In the fireball model, it is more physically realistic that ganuna-ray burst （GRB） ejecta have a range of bulk Lorentz factors （assuming M ∝ Г^-8）. The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases： the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sad ＆ Meszaros （2000）. However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.

关 键 词：GAMMA-RAYS bursts -- hydrodynamics -- radiation mechanisms nonther- mal -- shock waves 

分 类 号：O571.3[理学—粒子物理与原子核物理] TG659[理学—物理]