GRB 080928 afterglow imaging and spectro-polarimetry
GRB 080928 afterglow imaging and spectro-polarimetry
复制标题
GRB 080928 余辉成像和分光偏振测量
DOI：
10.1051/0004-6361/202243786
复制
发表时间：
2022
影响因子：
6.5
通讯作者：
Brivio R
中科院分区：
文献类型：
--
作者：
Brivio R
在我们可以观测到的各种各样的天体物理源中，伽马射线暴（GRBs）是整个宇宙中能量最高的。它们的辐射峰值在γ射线波段，持续时间从几分之一秒到几百秒不等，然后是覆盖整个电磁波谱的余辉。定义一个描述grb背后物理学的总体图景一直是一项引人注目的任务，但迄今为止从观测中获得的结果揭示了一个令人困惑的景象。由于缺乏清晰、独特的范例，因此需要进一步的观察和额外的、独立的技术。偏振法是一个非常有用的例子，因为它允许我们研究光源的一些特征，如发射区域的几何形状和磁场结构。迄今为止，只有少数空间望远镜探测到的爆发伴随着地面的光谱偏振跟踪，因此，为了增加多历元偏振分析的爆发样本，对更多grb进行这样的分析是至关重要的。在这项工作中，我们介绍了GRB 080928光学余辉的分析，并使用ESO-VLT FORS1仪器进行了观测。方法从成像偏振法（IPOL）和光谱偏振法（PMOS）采集的原始数据开始，进行数据还原，然后对爆发检测后~14和~40 h采集的IPOL数据进行光度分析，对PMOS数据进行光谱分析（t ~14.95 h）。通过计算描述发射辐射线性极化的简化Stokes参数sq /IandU/I，得到了三个观测历元的极化度。结果GRB光学余辉在第一个观测夜没有明显偏光。第二天晚上，极化度(P)增长到P ~4.5%的水平，证明了极化辐射在4σ置信水平上。GRB 080928的光曲线与标准的余辉模型并不完全一致，这使得与偏振模型的比较在一定程度上没有定论。最保守的解释是，GRB发射的特征是均匀喷流，观测角度为0.6 <θobs/θjet< 0.8。此外，第二晚的非零极化度表明在发射区存在一个占主导地位的局部有序磁场。
ContextAmong the large variety of astrophysical sources that we can observe, gamma-ray bursts (GRBs) are the most energetic of the whole Universe. Their emission peaks in theγ-ray band, with a duration from a fraction of a second to a few hundred seconds, and is followed by an afterglow covering the whole electromagnetic spectrum. The definition of a general picture describing the physics behind GRBs has always been a compelling task, but the results obtained so far from observations have revealed a puzzling landscape. The lack of a clear, unique paradigm calls for further observations and additional, independent techniques for this purpose. Polarimetry constitutes a very useful example as it allows us to investigate some features of the source such as the geometry of the emitting region and the magnetic field configuration.AimsTo date, only a handful of bursts detected by space telescopes have been accompanied by ground-based spectro-polarimetric follow-up, and therefore such an analysis of more GRBs is of crucial importance in order to increase the sample of bursts with multi-epoch polarisation analysis. In this work, we present the analysis of the GRB 080928 optical afterglow, with observations performed with the ESO-VLT FORS1 instrument.MethodsStarting from raw data taken in the imaging polarimetry (IPOL) and spectro-polarimetry (PMOS) modes, we performed data reduction, followed by the photometric analysis of IPOL data, taken ~14 and ~40 h after the burst detection, and spectroscopy of PMOS data (t ~14.95 h). After computing the reduced Stokes parametersQ/IandU/I, which describe the linear polarisation of the emitted radiation, we obtained the polarisation degree for the three observing epochs.ResultsWe find that the GRB optical afterglow was not significantly polarised on the first observing night. The polarisation degree (P) grew on the following night to a level ofP ~4.5%, giving evidence of polarised radiation at a 4σconfidence level. The GRB 080928 light curve is not fully consistent with standard afterglow models, making any comparison with polarimetric models partly inconclusive. The most conservative interpretation is that the GRB emission was characterised by a homogeneous jet and was observed at an angle of 0.6 <θobs/θjet< 0.8. Moreover, the non-zero polarisation degree on the second night suggests the presence of a dominant locally ordered magnetic field in the emitting region.