It is widely believed that many gamma-ray bursts are due to relativistic jets. Previous studies on beaming effects in gamma-ray bursts are mainly based on a conical geometry. However, some observations of relativistic jets in radio galaxies, active galactic nuclei, and "micro-quasars" have shown that many of these outflows are cylindrical, but not conical. In this study, we assume that the jets that produce gamma-ray bursts are cylindrical, and that the circum-burst environment is dense and optically thick. In the prompt burst phase, the strong X-ray emission can sublimate the dust in the circum-burst medium, generating an optically thin channel that allows optical photons to escape. As a result, optical afterglows can be observed only for observers that are on the axis of the jet. It is shown that the observed optical afterglows usually decay very rapidly (in the form of Sv ∝ t-p-1, where p is the power-law index of electrons), due to the joint effect of the lateral expansion of the cylindrical jet and the absorption of optical photons by dust outside the channel. Our model provides a possible explanation for the so called dark gamma-ray bursts.%很多伽玛射线暴应当是产生于极端相对论性的喷流.关于喷流,绝大多数的讨论都是围绕锥状喷流展开.然而有观测发现,一些天体中的喷流在很长距离上始终保持着几乎不变的截面积,即表现为柱状喷流.研究致密介质环境中有侧向膨胀的柱状喷流的余辉,描述其动力学演化和辐射过程,分别得到解析解和数值解,并对两者进行了对比.研究的暴周星际介质是光学厚的,在初始的主暴阶段,喷流辐射出高度准直的高能射线,升华了暴周介质,形成一个在光学波段光学薄的柱状通道.余辉阶段,由于喷流是有侧向膨胀的,观测者只能收集到视觉面积占比例越来越小的光学辐射,理论上可得到衰减极为快速的光变曲线,流量随时间的衰减约为Svα t-p-1(p为电子幂律分布的谱指数).如此迅速的衰减使得光学余辉将难以被观测到,提供了一种对暗伽玛射线暴的解释.
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