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arXiv 1109.3453astro-ph.HE
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风介质中的滑行外激波:雨燕GRB余辉中X射线平台衰变分量的起源

Coasting external shock in wind medium: an origin for the X-ray plateau decay component in Swift GRB afterglows

Rongfeng Shen, Christopher D. Matzner

摘要:

在大约一半的早期X射线余辉中观察到的平台是雨燕探测到的伽马射线暴(GRB)中最令人困惑的特征。通过分析一个大的X射线平台样品的时间指数和光谱指数,我们发现,55%可以解释为外部的,向前冲击同步辐射产生的相对论性喷出物在一个ρ~ r^{-2},风状介质中滑行,不需要能量注入到冲击。当喷出物聚集了足够的介质并过渡到绝热、减速的冲击波阶段后,它产生后平台衰变。对于符合这个模型的爆发,我们发现喷出物的初始洛伦兹因子有一个上限,即Γ_0 \leq 46(ε_e/0.1)^{-0.24}(ε_B/0.01)^{0.17};各向同性等效总喷出物能量为E_{iso} ~ 10^{53}(ε_e/0.1)^{-1.3}(ε_B/0.01)^{-0.09}(t_b/10^4 s)erg,其中ε_e和ε_B分别是激波下游总能量中由电子和磁场携带的部分,t_b是平台的终点。我们的发现支持沃尔夫-拉叶星是某些伽玛暴的祖先星。它提出了一个有趣的问题,一个中间-Γ_0喷出物的起源,我们推测这是连接到伽玛射线暴喷流出现从它的宿主恒星。对于剩余的45%的样本,后高原下降太快,无法解释在风中滑行模型,和能量注入似乎是必要的。

英文摘要:

The plateaus observed in about one half of the early X-ray afterglows are the most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By analyzing the temporal and spectral indices of a large X-ray plateau sample, we find that 55% can be explained by external, forward shock synchrotron emission produced by a relativistic ejecta coasting in a ρ~ r^{-2}, wind-like medium; no energy injection into the shock is needed. After the ejecta collects enough medium and transitions to the adiabatic, decelerating blastwave phase, it produces the post-plateau decay. For those bursts consistent with this model, we find an upper limit for the initial Lorentz factor of the ejecta, Γ_0 \leq 46 (ε_e/0.1)^{-0.24} (ε_B/0.01)^{0.17}; the isotropic equivalent total ejecta energy is E_{iso} ~ 10^{53} (ε_e/0.1)^{-1.3} (ε_B/0.01)^{-0.09} (t_b/10^4 s) erg, where ε_e and ε_B are the fractions of the total energy at the shock downstream that are carried by electrons and the magnetic field, respectively, and t_b is the end of the plateau. Our finding supports Wolf-Rayet stars as the progenitor stars of some GRBs. It raises intriguing questions about the origin of an intermediate-Γ_0 ejecta, which we speculate is connected to the GRB jet emergence from its host star. For the remaining 45% of the sample, the post-plateau decline is too rapid to be explained in the coasting-in-wind model, and energy injection appears to be required.

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