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arXiv 1303.7323astro-ph.HE
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用费米大面阵望远镜探测巨分子云中的宇宙射线

Probing Cosmic Rays in Nearby Giant Molecular Clouds with the Fermi Large Area Telescope

Rui-zhi Yang, Emma de Oña Wilhelmi, Felix Aharonian

摘要:

利用费米大天区望远镜(LAT)对古尔德带附近8个巨分子云(GMC)进行了为期5年的$γ$射线观测,研究了本星系中宇宙射线(CR)的能谱和绝对通量。获得的$γ$射线信号具有很高的统计显著性,使得能够以足够的精度确定300 MeV以上的$γ$射线谱,以提取这些云中CR的能量分布。值得注意的是,无论是派生的光谱指数和CR质子的绝对通量的能量间隔$10 - 100~ \rm \ GeV$同意最近的直接测量本地CR的PAMELA实验。这是一个强有力的证据,证明CR的分布相当均匀,至少在本星系的几百秒差距内。结合已确定的CR从银河系逃逸的能量依赖时间,$τ(E) \propto E^{-δ}$和$δ\approx 0.5-0.6$,测量的光谱意味着$\approx$ E $^{-2.3}$的(加速度)源的CR光谱指数。在低能量下,$γ$射线的光谱似乎因云而异。这意味着低于10~GeV的CR能谱的空间变化,在这样低的能量下,可以通过传播效应的影响和CR在云内局部加速的贡献来自然地解释。

英文摘要:

We report the results of our study of the energy spectra and absolute fluxes of cosmic rays (CRs) in the Local Galaxy based on a five-year $γ$-ray observation with the Fermi Large Area Telescope (LAT) of eight nearby giant molecular clouds (GMCs) belonging to the Gould Belt. The $γ$-ray signals obtained with high statistical significance allow the determination of $γ$-ray spectra above 300~MeV with adequate precision for extraction of the energy distributions of CRs in these clouds. Remarkably, both the derived spectral indices and the absolute fluxes of CR protons in the energy interval $10 - 100~ \rm \ GeV$ agree with the recent direct measurements of local CRs by the PAMELA experiment. This is strong evidence of a quite homogeneous distribution of CRs, at least within several hundred parsecs of the Local Galaxy. Combined with the well established energy-dependent time of escape of CRs from the Galaxy, $τ(E) \propto E^{-δ}$ with $δ\approx 0.5-0.6$, the measured spectrum implies a CR spectral index of the (acceleration) source of $\approx$E$^{-2.3}$. At low energies, the spectra of $γ$ rays appear to vary from one cloud to another. This implies spatial variations of the energy spectra of CRs below 10~GeV, which at such low energies could be explained naturally by both the impact of the propagation effects and the contribution of CR locally accelerated inside the clouds.

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