FERMI-LAT OBSERVATIONS OF THE DIFFUSE γ-RAY EMISSION: IMPLICATIONS FORCOSMIC RAYS AND THE INTERSTELLAR MEDIUM
FERMI-LAT OBSERVATIONS OF THE DIFFUSE γ-RAY EMISSION: IMPLICATIONS FORCOSMIC RAYS AND THE INTERSTELLAR MEDIUM
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DOI：
10.1088/0004-637x/750/1/3
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发表时间：
2012-05-01
影响因子：
4.9
通讯作者：
Zimmer, S.
中科院分区：
文献类型：
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作者：
Ackermann, M.;Ajello, M.;Zimmer, S.
大于100 MeV的伽马射线天空主要是宇宙射线与银河系的星际气体和辐射场相互作用产生的漫射辐射。对这些漫射辐射的观测为研究宇宙射线的起源和传播以及星际介质提供了一种工具。我们目前的测量从第一个21个月的费米大面积望远镜（费米-LAT）的使命和比较模型的扩散伽马射线发射产生的GALPROP代码。模型拟合宇宙射线数据，并纳入宇宙射线源，星际气体和辐射场分布的天体物理输入。为了评估与天体物理输入相关的不确定性，通过在观测极限内改变宇宙射线源的分布、宇宙射线约束体积（晕）的大小和星际气体的分布来创建模型网格。一个全天空的最大似然拟合被用来确定的X-CO因子，积分CO线强度和H-2柱密度之间的比率，从第一费米-LAT目录的伽马射线点源的通量和光谱，和强度和光谱的各向同性背景，包括残留的宇宙射线被误分类为伽马射线，所有这些都有一些依赖于假设的扩散发射模型。模型进行比较的基础上，他们的最大似然比，以及光谱，经度和纬度配置文件。我们还提供了残差图的数据减去扩散发射模型。该模型是一致的，在高纬度和中纬度的数据，但低估了在银河系内部的能量超过几个GeV的数据。这种差异的可能解释进行了讨论，包括未检测到的点源人口和整个银河系的宇宙射线光谱变化的贡献。在外星系，我们发现，数据更喜欢模型与平坦的宇宙射线源的分布，一个更大的宇宙射线晕，或更大的气体密度比通常假设。我们在外星系的研究结果是一致的，与其他费米-LAT研究这一地区，使用不同的分析方法比本文中所采用的。
The gamma-ray sky >100MeVis dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse gamma-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X-CO factor, the ratio between integrated CO-line intensity and H-2 column density, the fluxes and spectra of the gamma-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as gamma-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.