合并星系团A520中的弓形激波：射电晕的边缘和电子-离子平衡时标
Bow shock in merging cluster A520: the edge of the radio halo and the electron-ion equilibration timescale
摘要：
利用钱德拉深空X射线观测和VLA射电资料研究了合并星系团A520中的弓形激波。这种激波是一种有用的诊断工具，因为它具有清晰的几何形状和相对较高的马赫数。在激波的“头部”，我们测得马赫数为$M=2.4_{-0.2}^{+0.4}$。激波从合并轴开始倾斜，在距头部约30 $^{\circ}$处马赫数下降到$\simeq$ 1.6。紧挨激波鼻后的电子温度与朗肯-雨果尼奥实验的结果一致，并且高于（95%置信度）电子绝热压缩和随后库仑电子-质子平衡的预期，表明平衡机制的存在比库仑碰撞更快。这与之前对Bullet星系团的发现类似。我们还结合了四个档案VLA数据集，以获得1.4 GHz的星团巨型射电晕的更好图像。突然边缘的无线电晕跟踪冲击波前，并没有发射检测到的震前区域。如果射电边缘仅仅是由于激波前等离子体中相对论性电子的绝热压缩，我们可以预期在这个射电数据集中可以检测到激波前的射电辐射;然而，干涉伪影主导了不确定性，所以我们不能排除这个模型。射电晕的其他有趣的特征包括在冷核心的残余部分有一个峰值，表明核心曾经有一个射电微晕，还有一个峰值标志着可能的高湍流区域。
英文摘要：
We studied the prominent bow shock in the merging galaxy cluster A520 using a deep Chandra X-ray observation and archival VLA radio data. This shock is a useful diagnostic tool, owing to its clear geometry and relatively high Mach number. At the "nose" of the shock, we measure a Mach number of $M=2.4_{-0.2}^{+0.4}$. The shock becomes oblique away from the merger axis, with the Mach number falling to $\simeq$1.6 around 30$^{\circ}$ from the nose. The electron temperature immediately behind the shock nose is consistent with that from the Rankine-Hugoniot adiabat, and is higher (at a 95% confidence) than expected for adiabatic compression of electrons followed by Coulomb electron-proton equilibration, indicating the presence of equilibration mechanisms faster than Coulomb collisions. This is similar to an earlier finding for the Bullet cluster. We also combined four archival VLA datasets to obtain a better image of the cluster's giant radio halo at 1.4 GHz. An abrupt edge of the radio halo traces the shock front, and no emission is detected in the pre-shock region. If the radio edge were due only to adiabatic compression of relativistic electrons in pre-shock plasma, we would expect a pre-shock radio emission detectable in this radio dataset; however, an interferometric artifact dominates the uncertainty, so we cannot rule this model out. Other interesting features of the radio halo include a peak at the remnant of the cool core, suggesting that the core used to have a radio minihalo, and a peak marking a possible region of high turbulence.
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