内激波火球中e^±的产生、湮灭及再辐射

The Generation, Annihilation and the Re-radiation of the e± in the Internal Shocks

在内激波伽玛暴(GRB)模型下,中心能源喷出一系列质量相当但整体Lorentz因子相差悬殊的物质壳层,这些先后快慢的壳层发生激烈的碰撞并产生相对论性的激波,壳层中的电子被激波加热后通过同步辐射和逆康普顿散射发射高能γ光子.对于能量高达GeV的高能光子(观测者系)可能因为γ-γ碰撞产生电子对而被火球吸收.Pilla和Leob数值计算发现产生的电子对数目远高于火球本身的电子数目,Li等人最近也得到了类似的结果并以此来解释早期余辉中缺少光学闪.通过解析研究该过程中电子对的产生与湮灭随时间的演化后,发现:对于一个典型的pulse,同步高能部分产生的e±数目早期较多,湮灭率也高;在后期由于受到最大同步辐射频率的限制,该成分不再对e±的产生有贡献.与之不同,逆康普顿散射成分对e±的产生的贡献近似与pulse的持续时标成正比.在典型的参数范围下,两种成分共同作用产生的电子对数目可达原火球携带的电子数目的10来倍.由于所产生的e±的Lorentz因子较小,相应的同步辐射不会影响到观测谱(至少在BATSE探测器的能段是这样),但再次逆康普顿散射后则可能影响到观测谱.由于电子对的质量远比质子质量小,所以对后期的火球动力学演化的影响不大.至少对于均匀介质环境,电子对的存在对于早期余辉的光学辐射影响不大.

In the standard internal shocks model for the Gamma-ray bursts, the pulse is generated by the collision of two ultra-relativistic shells with variable Lorentz factors and comparable masses. The radiation mechanisms are believed to be synchrotron and inverse Compoton (IC). In this case, the heated shells are optically thin for the usual γ rays . However, for γ-rays with energy high up to tens GeV. they are optically thick. As a result, these photons annihilate with the soft photons and e± pairs are created. Pilla & Loeb have simulated this process numerically (1998) and shown that a large amount of e± have been generated. Recently, Li, Dai & Lu (2003) have tried to explain why there are only three optical flashes by considering the rich e± pairs contained in the fireball. In this paper, we focus on describing the generation and annihilation quantities as functions of the arrival time. We found that the high energy component of synchrotron radiation contributes more at the early time of the pulse, but at the later time, it ends up due to the restriction of the maximum synchrotron radiation frequency. On the contrary, the IC component contributes more at later time. In total, the IC component contribution is more than the high energy component of the synchrotron radiation. The re-radiation of the e± pairs have been investigated, and we have found that there is no significant emission at least in the BATSE energy band due to the low Lorentz factor of the generated e±pairs. However, the secondary IC process may play some role on the observed emission.