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arXiv 1310.0551astro-ph.HE
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衰减磁场作用下的伽马射线暴谱

Gamma-Ray Burst Spectrum with Decaying Magnetic Field

X. H. Zhao, Z. Li, X. W. Liu, B. B. Zhang, J. M. Bai, P. Meszaros

摘要:

在伽马射线暴的内激波模型中,由于下游均匀磁场中快冷电子的同步辐射谱太软,不能产生低能的伽玛暴谱。然而,磁场可能随着与冲击波阵面的距离而衰减。在这里,我们表明,同步辐射光谱变得更难,如果电子在衰减MF进行同步辐射和逆康普顿冷却。为了使其与具有低能量斜率$νF_ν\proptoν$的典型伽玛射线暴谱相一致,需要激波后MF衰变时间与体电子的冷却时间相当(对应于MF衰变长度,通常为$\sim10^5$趋肤深度);在MF衰变时间之后,逆康普顿冷却应该主导同步加速器冷却;和/或MF衰变的共动时间大致为$B\propto t^{-1.5}$。一个内激波同步加速器模型与衰减MF可以解释大多数的伽玛暴与低能量斜率不硬比$ν^{4/3}$。

英文摘要:

In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However the magnetic field may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low energy slope $νF_ν\proptoν$, it is required that the postshock MF decay time is comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of $\sim10^5$ skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as $B\propto t^{-1.5}$. An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low energy slopes not harder than $ν^{4/3}$.

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