标题： HI柱密度 z > 2 的Swift伽马射线暴 显示英文标题

标题： HI column densities of z > 2 Swift gamma-ray bursts

摘要： 上下文：在Swift卫星发射之前，大多数伽马射线暴（GRB）余辉的Ly-alpha因红移进入可观测光谱，显示出存在阻尼Ly-alpha吸收体的证据。这个小样本表明，GRB要么在星系中爆炸，要么在星系内的区域爆炸，这些区域具有高中性氢柱密度。目的：增加红移大于2的GRB的光谱样本，从而确定沿GRB视线方向的N(HI)分布。方法：我们获得了六个红移大于2的GRB余辉光谱，并在每种情况下拟合了Ly-alpha吸收线以确定N(HI)。这还补充了文献中其他12个Swift N(HI)值。结果：我们显示GRB N(HI)分布的峰值在定性上与GRB起源于类似银河系的分子云的模型一致。然而，系统性差异，特别是与预测相比低柱密度系统的过剩，表明选择效应和云内条件（例如强电离）会影响观测到的N(HI)范围。我们还报告了GRB 060714宿主的Ly-alpha发射，对应于大约0.8 M_Sun/年的恒星形成率。最后，我们给出了六个爆发的精确红移：z = 3.240 +/- 0.001（GRB 050319），z = 2.198 +/- 0.002（GRB 050922C），z = 3.221 +/- 0.001（GRB 060526），z = 3.425 +/- 0.002（GRB 060707），z = 2.711 +/- 0.001（GRB 060714）和z = 3.686 +/- 0.002（GRB 060906）。 显示英文摘要

摘要： Context: Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Ly-alpha was redshifted into the observable spectrum showed evidence for a damped Ly-alpha absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities. Aims: To increase the spectroscopic sample of GRBs with z > 2 and hence establish the N(HI) distribution along GRB lines-of-sight. Methods: We have obtained six z > 2 GRB afterglow spectra and fitted the Ly-alpha absorption line in each case to determine N(HI). This has been complemented with 12 other Swift N(HI) values from the literature. Results: We show that the peak of the GRB N(HI) distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed N(HI) range. We also report the discovery of Ly-alpha emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 M_Sun/year. Finally, we present accurate redshifts of the six bursts: z = 3.240 +/- 0.001 (GRB 050319), z = 2.198 +/- 0.002 (GRB 050922C), z = 3.221 +/- 0.001 (GRB 060526), z = 3.425 +/- 0.002 (GRB 060707), z = 2.711 +/- 0.001 (GRB 060714) and z = 3.686 +/- 0.002 (GRB 060906).