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arXiv 2002.04614astro-ph.HEastro-ph.SR
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伽马射线暴外激波中磁场几何的极化约束

Polarization constraints on the geometry of the magnetic field in the external shock of gamma-ray bursts

Eric Stringer, Davide Lazzati

摘要:

研究了长持续时间伽马射线暴光学余辉中线偏振测量的系综。我们假设一个非侧向扩展的顶帽射流几何形状,并使用相对大量的测量下,假设他们代表一个统计无偏的样本。这使我们能够限制最大预测极化和测量极化之间的比率,这是外部冲击下游区域磁场几何形状的指标。我们发现,所测量的极化基本上被抑制相对于最大可能的一个完全有序的磁场平行于冲击法线或一个领域,这是完全包含在冲击平面。平均而言,测量的偏振被限制在最大理论可能值的25%和30%之间。这种减小要求磁场的垂直分量相对于平行于冲击波阵面的分量在能量上占主导地位,如对于冲击产生和/或冲击压缩场所预期的那样。然而,我们发现,数据只勉强支持一个简单的礼帽射流的假设,指向一个更复杂的几何外流。

英文摘要:

We study the ensemble of linear polarization measurement in the optical afterglows of long-duration gamma-ray bursts. We assume a non sideways-expanding top-hat jet geometry and use the relatively large number of measurements under the assumption that they represent a statistically unbiased sample. This allows us to constrain the ratio between the maximum predicted polarization and the measured one, which is an indicator of the geometry of the magnetic field in the downstream region of the external shock. We find that the measured polarization is substantially suppressed with respect to the maximum possible for either a completely ordered magnetic field parallel to the shock normal or to a field that is entirely contained in the shock plane. The measured polarization is limited, on average, to between 25 and 30% of the maximum theoretically possible value. This reduction requires the perpendicular component of the magnetic field to be dominant in energy with respect to the component parallel to the shock front, as expected for a shock generated and/or shock compressed field. We find, however, that the data only marginally support the assumption of a simple top-hat jet, pointing towards a more complex geometry for the outflow.

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