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arXiv 1002.4101astro-ph.COastro-ph.HE
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z ~ 6.3时，GRB 050904中无粉尘消失的证据

No evidence for dust extinction in GRB 050904 at z ~ 6.3

Tayyaba Zafar, Darach J. Watson, Daniele Malesani, Paul M. Vreeswijk, Johan P. U. Fynbo, Jens Hjorth, Andrew J. Levan, Michał J. Michałowski

备注：
7 pages, 3 figures; accepted for publication in A&A
发表信息：
A&A 515, A94 (2010)
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摘要：

背景：伽玛射线暴余辉是恒星形成星系中气体和尘埃的极好探测器。人们已经假定，早期宇宙中的尘埃必须与z较低的尘埃不同。到目前为止，有两份报告直接支持这一论点，其中一份是基于GRB 050904在z = 6.295处的光谱形状。目的：我们重新研究余辉，以了解在高z尘埃。我们通过同时检查X射线和光学/近红外分光光度数据来解决其宿主星系中不寻常（SN起源）尘埃的证据。研究方法：我们得出的余辉在0.47，1.25和3.4天的固有SED，通过重新减少斯威夫特X射线数据，1.25天FORS 2 z-耿氏光度数据，光谱和z '波段光度数据在约3天的斯巴鲁望远镜，以及关键UKIRT Z波段光度在0.47天，在很大程度上依赖于尘埃探测的索赔。结果：我们没有发现在SED的灰尘灭绝的证据。我们计算通量密度在lambda_rest = 1250 AA直接从观察到的计数在所有时期。在最早的时期，0.47天，Z波段的抑制被发现比以前报道的要小（0.3 ± 0.2等），并且在统计上不显著（<1.5西格玛）。此外，我们还发现这一波段的光度测量是不稳定的，而且很难校准。结论：从余辉SED中，我们证明了没有尘埃消光的证据-在任何时候都可以在没有尘埃的情况下重现SED，特别是在1.25天，可以排除显著的消光，使用SN型消光曲线，A（3000 AA）< 0.27 mag，置信度为95%。我们的结论是，没有证据表明GRB 050904的余辉中存在任何消光现象，GRB 050904宿主中存在SN源尘埃必须持怀疑态度。[删节]

英文摘要：

Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z'-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction -- the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]

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