Bow shock and radio halo in the merging cluster A520
Bow shock and radio halo in the merging cluster A520
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DOI：
10.1086/430695
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发表时间：
2005-07-10
影响因子：
4.9
通讯作者：
Jerius, D
中科院分区：
文献类型：
--
作者：
Markevitch, M;Govoni, F;Jerius, D
钱德拉对合并星系团A520的观测显示了一个显著的弓形激波，M = 2.1(-0.3)（+0.4）。这只是集群中实质上超音速合并冲击前沿的第二个明显例子。将x射线图像与先前已知的射电晕的图像进行比较，揭示了晕的前缘与弓形激波的巧合，为确定激波在射电晕产生中的作用提供了一个有趣的实验装置。A520中的光晕显然由两个空间上不同的部分组成，即主要的湍流驱动部分和与激波相关的帽状正向结构，后者可能为随后的湍流再加速提供预激励电子。无线电边缘可能是由激波引起的电子加速引起的。如果是这样，同步加速器光谱在边缘后面应该有一个近似或等于1.2的斜率，在远离边缘的地方会迅速变陡。或者，如果激波是低效的加速器，无线电边缘可能是由于气体压缩导致磁场和先前存在的相对论性电子密度的增加而解释的。在后一种模型中，激波前应该有射电发射，其光谱与激波后相同，但亮度要弱10-20倍。如果未来灵敏的无线电测量没有发现这种冲击前发射，那么电子确实被冲击加速（或再加速），人们将能够确定它的加速效率。我们还提出了一种估算激波后磁场强度的方法，该方法基于测量与激波距离有关的无线电频谱斜率。此外，无线电边缘提供了一种限制相对论性电子扩散速度的方法。
Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M = 2.1(-0.3)(+0.4). This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide preenergized electrons for subsequent turbulent reacceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of alpha similar or equal to 1.2 right behind the edge, with quick steepening farther away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of preexisting relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such preshock emission, then the electrons are indeed accelerated ( or reaccelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.