Low-Resolution Spectroscopy of Gamma-ray Burst Optical Afterglows: Biases in the Swift Sample and Characterization of the Absorbers
J. P. U. Fynbo, P. Jakobsson, J. X. Prochaska, D. Malesani, C. Ledoux, A. de Ugarte Postigo, M. Nardini, P. M. Vreeswijk, K. Wiersema, J. Hjorth, J. Sollerman, … 展开作者
J. P. U. Fynbo, P. Jakobsson, J. X. Prochaska, D. Malesani, C. Ledoux, A. de Ugarte Postigo, M. Nardini, P. M. Vreeswijk, K. Wiersema, J. Hjorth, J. Sollerman, H. -W. Chen, C. C. Thoene, G. Bjoernsson, J. S. Bloom, A. Castro-Tirado, L. Christensen, A. De Cia, A. S. Fruchter, J. U. Gorosabel, J. F. Graham, A. Jaunsen, B. L. Jensen, D. A. Kann, C. Kouveliotou, A. Levan, J. Maund, N. Masetti, B. Milvang-Jensen, E. Palazzi, D. A. Perley, E. Pian, E. Rol, P. Schady, R. Starling, N. Tanvir, D. J. Watson, D. Xu, T. Augusteijn, F. Grundahl, J. Telting, P. -O. Quirion
(Abridged). We present a sample of 77 optical afterglows (OAs) of Swift detected GRBs for which spectroscopic follow-up observations have been secured. We provide linelists and equivalent widths for all detected lines redward of Ly-alpha. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray selected sample of Swift bursts with optimal conditions for ground-based follow up from the period March 2005 to September 2008; 146 bursts fulfill our sample criteria. We derive the redshift distribution for this sample and conclude that less than 19% of Swift bursts are at z>7. We compare the high energy properties for three sub-samples of bursts in the sample: i) bursts with redshifts measured from OA spectroscopy, ii) bursts with detected OA, but no OA-based redshift, and iii) bursts with no detection of the OA. The bursts in group i) have significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fraction of dark bursts is 14% in group i), 38% in group ii) and > 39% in group iii). From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight-lines. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular DLAs. On average GRB absorbers are characterized by significantly stronger EWs for HI as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. Based on the z>2 bursts in the sample we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies.
Casting light on the 'anomalous' statistics of Mg II absorbers toward Gamma-Ray Burst afterglows: the incidence of weak systems
我们重新访问阶梯光谱（光谱分辨率R ~ 40000）的8个伽马射线暴余辉，以获得的发病率（dN/dz）弱干预Mg II系统在平均红移<z>= 1.5。我们发现，dN/dz的系统具有restframe等效宽度0.07 A < W_r（Mg Ⅱ）< 1 A toward GRBs is statistically consistent with the incidence toward QSOs. Our result is in contrast to the results for Mg II systems having W_r >1 A，其中的dN/dz对伽玛暴已被发现是大于对类星体的一个因素~ 4。我们证实了密度过高，尽管只有~ 3倍。这表明，任何对GRB/QSO差异的解释，无论是吸收体本身还是选择效应，都应该是只有在GRB视线内拥有强吸收体的星系才固有的。我们认为，在所有已提出的方案，透镜放大是一个可以解释强镁II增强，而不允许在弱吸收剂的显着增强。
GRB 070125 and the environments of spectral-line poor afterglow absorbers
GRB 070125是迄今为止探测到的能量最高的爆发之一，也是观测最广泛的爆发之一。然而，在余辉物理学和伽玛射线暴环境的文献中，尚未解决的问题仍然是开放的。特别是，GRB 070125被认为是在星系晕环境中爆炸的，这是基于光谱的独特性和没有探测到潜在的宿主星系。在这项工作中，我们收集了所有公开可用的数据，并解决这些问题，通过建模的NIR到X射线光谱能量分布（SED）和研究的高信号噪声VLT/FORS余辉光谱相比，一个更大的样本的伽玛射线暴吸收剂。SED揭示了同步辐射冷却中断的紫外线，低当量氢柱密度和小变红所造成的LMC或SMC型消光曲线。从光谱中z=1.5477处的弱MgII吸收，我们推导出logN（MgII）=12.96+0.13-0.18和几种金属离子柱密度的上限。这表明伽玛射线暴吸收体很可能是一个金属丰度为0.03 Z \odot <Z<1.3 Z \odot的LLS。与其他GRB吸收体的比较，GRB 070125的位置在低端的吸收线等效宽度分布，确认弱的光谱特征和谱线差的吸收体是不是那么罕见的余辉光谱。此外，我们表明，光电离对周围的GRB的气体的影响，结合低N（HI）沿短段的视线内的宿主星系，可以解释GRB 070125光谱特征的缺乏。最后，未探测到的底层星系与微弱的GRB宿主星系一致，在GRB宿主亮度分布范围内。因此，GRB 070125可能只是位于其小而微弱的宿主星系内…
