An unexpectedly low oscillator strength as the origin of the Fe XVII emission problem
An unexpectedly low oscillator strength as the origin of the Fe XVII emission problem
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DOI：
10.1038/nature11627
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发表时间：
2012-12-13
期刊：
影响因子：
64.8
通讯作者：
Lopez-Urrutia, J. R. Crespo
中科院分区：
文献类型：
--
作者：
Bernitt, S.;Brown, G. V.;Lopez-Urrutia, J. R. Crespo
高电荷铁（Fe16+，此处称为 Fe XVII）会从热天体物理物体 (1)（包括星系团和恒星日冕）产生一些最亮的 X 射线发射线，并且它主导着太阳在 15 埃附近波长的发射。然而，即使是最好的天体物理模型也很难拟合 Fe XVII 光谱。一个特殊的问题是最强的 Fe XVII 谱线的强度通常弱于预测 (2,3)。这影响了钱德拉和 XMM-牛顿轨道 X 射线任务 (1) 观测结果的解释，引发了关于这种差异是否是由于这些物体中等离子体环境的不完整建模或基础原子物理学处理缺陷造成的持续争论。在这里，我们报告了一项实验的结果，其中通过将铁离子目标置于自由电子激光器的飞秒 X 射线脉冲下，使其发出荧光 (4)；我们的目标是分离出线发射的量子力学描述的一个关键方面。令人惊讶的是，我们发现相对振荡器强度出乎意料地低，与最佳量子力学计算相差 3.6 西格玛。我们的测量表明，不一致的根源在于基础原子波函数的质量，而不是碰撞过程的建模不足。
Highly charged iron (Fe16+, here referred to as Fe XVII) produces some of the brightest X-ray emission lines from hot astrophysical objects(1), including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 angstroms. The Fe XVII spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe XVII line is generally weaker than predicted(2,3). This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions(1), fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser(4); our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6 sigma from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.