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arXiv 0806.3607astro-ph
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短持续时间和长持续时间伽马射线暴余辉的比较

A Comparison of the Afterglows of Short- and Long-Duration Gamma-Ray Bursts

M. Nysewander, A. S. Fruchter, A. Pe'er

摘要:

本文对短、长持续时间伽马射线暴(GRB)的光学余辉和X射线余辉的观测特性进行了比较研究。使用37个短和421个长伽玛暴的大样本,我们发现11小时后测量的余辉亮度和观测到的即时发射通量之间有很强的相关性。光学(R波段)和X射线通量密度(F_R和F_X)都与γ射线注量F_γ成比例。对于具有已知红移的爆发,在11小时(静止坐标系)的余辉通量密度和总的各向同性伽马射线能量Egi之间存在紧密的相关性:F_{R,X} ~ Egi^{alpha},其中alpha ~ 1。当Egi是从雨燕的数据中得到时,长爆发和短爆发的比例常数几乎是相同的。此外,我们还发现,对于F_gamma >10^{-7} erg cm^{-2}的短爆发,光学余辉几乎总是被相当深的早期观测所探测到。最后,我们证明了短爆发和长爆发的F_R/F_X比值非常相似。这些结果很难用标准的理论框架来解释,因为它们要求:(1)短暴周围介质的数密度与长暴的数密度相当,甚至更大;(2)短暴爆发成一个密度分布,n(r)αr^{-2};或者(3)瞬发伽马射线注量取决于外部介质的密度。因此,我们发现很可能要么是对短伽玛暴环暴环境性质的基本假设,要么是对伽玛暴发射的标准模型必须重新进行检验。我们认为,最可能的解决办法是,典型短暴周围的环境密度比通常预期的要高:典型值约为1/cm^{-3}。

英文摘要:

We present a comparative study of the observed properties of the optical and X-ray afterglows of short- and long-duration gamma-ray bursts (GRBs). Using a large sample of 37 short and 421 long GRBs, we find a strong correlation between the afterglow brightness measured after 11 hours and the observed fluence of the prompt emission. Both the optical (R band) and X-ray flux densities (F_R and F_X) scale with the gamma-ray fluence, F_gamma. For bursts with a known redshift, a tight correlation exists between the afterglow flux densities at 11 hours (rest-frame) and the total isotropic gamma-ray energy, Egi: F_{R,X} ~ Egi^{alpha}, with alpha ~ 1. The constant of proportionality is nearly identical for long and short bursts, when Egi is obtained from the Swift data. Additionally, we find that for short busts with F_gamma >10^{-7} erg cm^{-2}, optical afterglows are nearly always detected by reasonably deep early observations. Finally, we show that the ratio F_R/F_X has very similar values for short and long bursts. These results are difficult to explain in the framework of the standard scenario, since they require that either (1) the number density of the surrounding medium of short bursts is typically comparable to, or even larger than the number density of long bursts; (2) short bursts explode into a density profile, n(r)αr^{-2} or (3) the prompt gamma-ray fluence depends on the density of the external medium. We therefore find it likely that either basic assumptions on the properties of the circumburst environment of short GRBs or else the standard models of GRB emission must be re-examined. We believe that the most likely solution is that the ambient density surrounding typical short bursts is higher than has generally been expected: a typical value of ~1 per cm^{-3} is indicated.

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