The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo
The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo
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DOI：
10.1038/s41550-018-0593-y
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发表时间：
2018-09
期刊：
影响因子：
14.1
通讯作者：
M. Bulla;S. Covino;K. Kyutoku;Masaomi Tanaka;J. Maund;F. Patat;K. Toma;K. Wiersema;K. Wiersema-K.-Wi
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作者：
M. Bulla;S. Covino;K. Kyutoku;Masaomi Tanaka;J. Maund;F. Patat;K. Toma;K. Wiersema;K. Wiersema-K.-Wi
引力波事件GW 170817是由两颗中子星合并产生的，并产生了一个电磁瞬变，标记为AT 2017 gfo，这是大规模观测活动的目标。偏振测量是探测未分辨源的几何结构和发射过程的有力诊断工具，并且观察到的该事件的线性偏振与主要由介入尘埃引起的一致，表明本征发射是弱偏振的（P< 0.4-0.5%）。在这里，我们提出了一个详细的分析，预计从合并的中子星双星系统的3D蒙特卡罗辐射传输模拟假设一系列可能的配置，波长，历元和视角。我们发现，偏振起源于不均匀的不透明度分布内的喷出物，并可以达到1%的水平，在早期（合并后的一到两天），并在光学R波段。较小的偏振信号预计在稍后的时期和不同的波长。根据极化信号的视角依赖性，我们将AT 2017 gfo的观察者方向限制在与极方向约65°的范围内。在未来的事件中，非零偏振的检测将明确揭示不含镧系元素的喷出物成分的存在，并揭示其空间和角度分布。
The gravitational-wave event GW 170817 was generated by the coalescence of two neutron stars and produced an electromagnetic transient, labelled AT 2017gfo, that was the target of a massive observational campaign. Polarimetry is a powerful diagnostic tool for probing the geometry and emission processes of unresolved sources, and the observed linear polarization for this event was consistent with being mostly induced by intervening dust, suggesting that the intrinsic emission was weakly polarized (P< 0.4–0.5%). Here we present a detailed analysis of the linear polarization expected from a merging neutron-star binary system by means of 3D Monte Carlo radiative transfer simulations assuming a range of possible configurations, wavelengths, epochs and viewing angles. We find that polarization originates from the non-homogeneous opacity distribution within the ejecta and can reach levels of 1% at early times (one to two days after the merger) and in the optical R band. Smaller polarization signals are expected at later epochs and different wavelengths. From the viewing-angle dependence of the polarimetric signal, we constrain the observer orientation of AT 2017gfo to within about 65° from the polar direction. The detection of non-zero polarization in future events will unambiguously reveal the presence of a lanthanide-free ejecta component and unveil its spatial and angular distribution.