用六年Fermi-LAT资料寻找后发座星系团的gamma-ray辐射
Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data
摘要：
本文介绍了后发座星系团的γ射线观测结果，结合了6年的Fermi-LAT数据和新发布的{ \emph{Pass 8} }事件级分析。我们对该区域的分析揭示了该星系团维里半径内的低显著性残留结构，这些结构太微弱，无法用当前数据进行详细调查。使用的可能性的方法，是免费的光谱形状的假设，我们得到的γ射线通量的上限，预计从高能粒子相互作用的集群。我们还考虑了一个基准的空间和光谱模板的动机模型，其中所观察到的无线电晕主要是发射的次级电子。在这种情况下，100 MeV以上通量的预期和观测上限中值分别为$1.7\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$和$5.2\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$（后者对应于1.8σ水平的剩余发射）。这些界限相当于或高于预测水平的强子伽马射线发射的宇宙射线模型，或没有再加速的次级电子，虽然直接比较是敏感的假设宇宙射线和磁场属性的起源和传播模式。后发座星系团内射电和恒星形成星系的最小预期γ射线通量大约比我们分析的中值灵敏度低一个数量级。
英文摘要：
We present results from γ-ray observations of the Coma cluster incorporating 6 years of Fermi-LAT data and the newly released {\emph{Pass 8}} event-level analysis. Our analysis of the region reveals low-significance residual structures within the virial radius of the cluster that are too faint for a detailed investigation with the current data. Using a likelihood approach that is free of assumptions on the spectral shape we derive upper limits on the γ-ray flux that is expected from energetic particle interactions in the cluster. We also consider a benchmark spatial and spectral template motivated by models in which the observed radio halo is mostly emission by secondary electrons. In this case, the median expected and observed upper limits for the flux above 100 MeV are $1.7\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ and $5.2\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ respectively (the latter corresponds to residual emission at the level of 1.8σ). These bounds are comparable to or higher than predicted levels of hadronic gamma-ray emission in cosmic-ray models with or without reacceleration of secondary electrons, although direct comparisons are sensitive to assumptions regarding the origin and propagation mode of cosmic rays and magnetic field properties. The minimal expected γ-ray flux from radio and star-forming galaxies within the Coma cluster is roughly an order of magnitude below the median sensitivity of our analysis.
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