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arXiv 1402.2301astro-ph.COastro-ph.HE
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星系团X射线叠加谱中一条未知发射线的探测

Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters

Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall

备注：
Accepted for publication in ApJ, 27 pages, 15 figures. ApJ version has the abridged abstract and Figure 6 in the ApJ version has only the XMM MOS spectrum of the full sample
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摘要：

我们在73个星系团的XMM叠加谱中探测到一条E=（3.55-3.57）+/-0.03 keV的微弱的未识别发射线，红移范围为0.01-0.35。MOS和PN观测结果分别显示了该线在一致能量下的存在。当整个样本被分成三个子样本（英仙座，半人马座+蛇夫座+后发座，和所有其他），该线是显着检测在所有三个独立的MOS光谱和PN“所有其他”的光谱。它也在英仙座的钱德拉光谱中被检测到，其通量与XMM一致（尽管在室女座没有看到）。然而，它非常弱，位于几条已知的微弱谱线的50- 110 eV范围内，因此受到显著的建模不确定性的影响。关于这条线的起源，我们认为在这个能量下热等离子体中不应该有原子跃迁。一个有趣的可能性是无菌中微子的衰变，这是一种长期寻找的暗物质候选粒子。假设所有的暗物质都是m_s= 2 E =7.1 keV的无菌中微子，我们在整个样品中的探测结果对应于中微子衰变混合角sin ^2（2 θ）= 7 e-11，低于之前的上限。然而，基于星团的质量和距离，英仙座的线比这个模型中预期的要亮得多。这似乎是因为英仙座在E=3.62 keV处有一条异常明亮的线，可能是Ar XVII双电子复合线，尽管它的通量是预期值的30倍，物理上很难理解。原则上，这种异常也可以解释我们在其他子样本中的线检测，尽管它会扩大线能量的不确定性。另一种选择是Ar线中的上述异常与附近的3.51 keV K线相结合，也超出预期10-20倍。与钱德拉和朱雀，并最终Astro-H的确认，需要确定这条新线路的性质。（简略）

英文摘要：

We detect a weak unidentified emission line at E=(3.55-3.57)+/-0.03 keV in a stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01-0.35. MOS and PN observations independently show the presence of the line at consistent energies. When the full sample is divided into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is significantly detected in all three independent MOS spectra and the PN "all others" spectrum. It is also detected in the Chandra spectra of Perseus with the flux consistent with XMM (though it is not seen in Virgo). However, it is very weak and located within 50-110eV of several known faint lines, and so is subject to significant modeling uncertainties. On the origin of this line, we argue that there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark matter is in sterile neutrinos with m_s=2E=7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin^2(2theta)=7e-11, below the previous upper limits. However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model. This appears to be because of an anomalously bright line at E=3.62 keV in Perseus, possibly an Ar XVII dielectronic recombination line, although its flux would be 30 times the expected value and physically difficult to understand. In principle, such an anomaly might explain our line detection in other subsamples as well, though it would stretch the line energy uncertainties. Another alternative is the above anomaly in the Ar line combined with the nearby 3.51 keV K line also exceeding expectation by factor 10-20. Confirmation with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new line.(ABRIDGED)

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