X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic field near the shock, where we derive a magnetic-field amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.

超新星遗迹通常被认为是通过扩散激波加速产生了大部分银河系宇宙射线。然而，关于激波前沿的物理条件，例如靠近粒子加速位点的磁场形态等许多问题，仍然悬而未决。在此我们报告由X射线成像偏振探测器在第谷超新星遗迹的一些同步辐射X射线发射区域探测到局部偏振信号。推导出的X射线同步辐射发射的偏振度在整个遗迹上平均为9%±2%，在边缘为12%±2%，高于在射电波段观测到的7% - 8%的偏振值。在西部区域，偏振度为23%±4%。第谷超新星遗迹的X射线偏振度高于仙后座A，这表明磁场更有序或最大湍流尺度更大。测量到的偏振切向方向对应于径向磁场，并且与在射电波段观测到的一致。这些结果与激波附近径向磁场的各向异性级联所产生的湍流预期相符，我们由此推导出磁场放大因子为3.4±0.3。这个值明显小于加速模型所预期的值这一事实表明磁场湍流具有高度各向异性，或者发射电子要么倾向于湍流较低的区域，要么由于流体动力学不稳定性而聚集在磁场方向优先呈径向的区域附近。