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arXiv 0811.3443astro-ph
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GRB光学余辉和红移选择效应:工作中的学习曲线效应

GRB optical afterglow and redshift selection effects: The learning curve effect at work

D. M. Coward

摘要:

我们展示了如何观察到的伽马射线暴(GRB)的光学余辉(OA)和红移分布随时间变化的选择效应。对于一个子集的{ \it斯威夫特}触发的长持续时间的爆发,我们表明,所需的平均时间,以获取光谱红移的伽玛暴OA已演变到更短的时间。我们确定了一个很强的相关性之间的平均时间,以获得光谱红移和测量的红移。这种相关性表明,较短的响应时间有利于较小的红移爆发。这是一个令人信服的证据,选择效应,偏见较长的反应时间与相对较亮的高红移爆发。相反,对于较短的响应时间,相对较近的光学微弱的爆发足够明亮,可以获得光谱红移。这种选择效应可以解释为什么2005年测得的平均红移值$<z>\approx2.8$到2008年中期已经进化到了$<z>\approx2$。了解这些选择效应提供了一个重要的工具,分离的贡献,本质上微弱的爆发,那些模糊的主机星系尘埃和爆发中看不到的光学,因为他们的OAs观察到在较晚的时间。这项研究强调了快速响应望远镜能够光谱学的重要性,并确定了一个新的红移选择效应,这是以前没有考虑过的,即测量红移的响应时间。

英文摘要:

We show how the observed gamma ray burst (GRB) optical afterglow (OA) and redshift distributions are changing in time from selection effects. For a subset of {\it Swift} triggered long duration bursts, we show that the mean time taken to acquire spectroscopic redshifts for a GRB OA has evolved to shorter times. We identify a strong correlation between the mean time taken to acquire a spectroscopic redshift and the measured redshift. This correlation reveals that shorter response times favour smaller redshift bursts. This is compelling evidence for a selection effect that biases longer response times with relatively brighter high redshift bursts. Conversely, for shorter response times, optically fainter bursts that are relatively closer are bright enough for spectroscopic redshifts to be acquired. This selection effect could explain why the average redshift, $<z>\approx2.8$ measured in 2005, has evolved to $<z>\approx2$, by mid 2008. Understanding these selection effects provides an important tool for separating the contributions of intrinsically faint bursts, those obscured by host galaxy dust and bursts not seen in the optical because their OAs are observed at late times. The study highlights the importance of rapid response telescopes capable of spectroscopy, and identifies a new redshift selection effect that has not been considered previously, namely the response time to measure the redshift.

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