The slow decay of some radio afterglows – a puzzle for the simplest γ-ray burst fireball model
The slow decay of some radio afterglows – a puzzle for the simplest γ-ray burst fireball model
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一些射电余辉的缓慢衰减——最简单的γ射线爆发火球模型之谜
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发表时间：
2003
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通讯作者：
A. Kumar
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作者：
A. Kumar
在长时间监测下，一半的γ射线暴（GRB）射电余辉的衰减明显慢于光学频率，这与最简单的火球模型的预期相反。我们研究了四种方法来解耦的无线电和光学衰变：一个不断变化的指数的幂律分布的冲击加速的电子，两个域之间的光谱中断的存在下，一个结构化的外流，和一个长寿的反向冲击贡献的余辉发射。对于大多数余辉，第一种情况不能容纳余辉发射的所有属性。如果第二种情况下的光谱中断是冷却频率，那么观测结果要求，随着火球减速，最小电子能量的参数减小，磁场强度不变或增加。后一种现象似乎是不可信的，如果环爆流密度向外增加，这种现象就会得到缓解。在第三种情况下，光学余辉来自一个更有活力的，狭窄的外流核心，而无线电发射来自一个更广泛的信封。这种情况最多与无线电和光学余辉发射的一般性质勉强一致，并且需要超过均分的总电子能量，因此它不能提供可接受的解决方案。在第四种情况下，它是假设的无线电余辉发射出现在反向冲击传播的稳定流的喷出物，赶上减速的伽玛暴残留。这种情况下，可以容纳的余辉与缓慢的无线电衰减的属性，并要求注入的能量小于或相当于初始火球的能量，而其他余辉参数有合理的值。我们发现反向正向冲击的情况下，是最可行的解释，在一些无线电余辉中观察到的浅衰减。对于喷流，向半相对论运动的转变减轻了射电衰减;然而，这种情况只有在陡峭的光学衰减之后观察到较慢的射电衰减时才有效。一个具有相对论性核心的结构外流，产生快速衰减的光辐射，和一个非相对论性的包络，产生缓慢下降的无线电余辉，不是一个可行的解决方案，因为它不能解耦无线电和光衰减。
The decay of half of the γ-ray burst (GRB) radio afterglows with long temporal monitoring is significantly slower than at optical frequencies, contrary to what is expected in the simplest fireball model. We investigate four ways to decouple the radio and optical decays: an evolving index of the power-law distribution of the shock-accelerated electrons, the presence of a spectral break between the two domains, a structured outflow, and a long-lived reverse shock contribution to the afterglow emission. For most afterglows, the first scenario cannot accommodate all the properties of the afterglow emission. If the spectral break of the second scenario is the cooling frequency, then observations require that, as the fireball decelerates, the parameter for the minimal electron energy decreases and the magnetic field strength is constant or increases. The latter behaviour seems implausible and is alleviated if the circumburst density increases outwards. In the framework of the third scenario, the optical afterglow arises from a more energetic, narrow outflow core while the radio emission comes from a more extended envelope. This scenario is at best marginally consistent with the general properties of the radio and optical afterglow emissions and requires a total electron energy exceeding equipartition, and thus it does not provide an acceptable solution. In the fourth scenario, it is assumed that the radio afterglow emission arises in the reverse shock propagating in a steady stream of ejecta, which catch up with the decelerating GRB remnant. This scenario can accommodate the properties of the afterglows with slow radio decays and requires that the injected energy is less than or comparable to the initial fireball energy, while other afterglow parameters have reasonable values. We find the reverse-forward shock scenario to be the most viable explanation for the shallow decay observed in some radio afterglows. For a jet, the transition to a semirelativistic motion mitigates the radio decay; however, this scenario would work only when the slower radio decay is observed well after the steeper optical fall-off. A structured outflow with a relativistic core, yielding a fast-decaying optical emission, and a non-relativistic envelope, producing a slowly falling-off radio afterglow, is not a viable solution, as it fails to decouple the radio and optical decays.