The Fermi-LAT telescope has measured the extragalactic gamma-ray background (EGB) generated by the ensemble of all extragalactic sources. The energy distribution of the EGB is well described as a power-law with a spectral index approximately equal to 2.3, and an exponential cutoff, that is consistent with being the effect of absorption of high energy photons assuming an emission that is an unbroken power-law spectrum. The observations of the Fermi telescope have also resolved the EGB, determining that most of it is formed by the sum of the contributions of Active Galactic Nuclei (AGN) of the blazar class. The spectra of the individual AGN sources have a broad range of spectral shapes, and the brightest and most precisely measured sources have curved spectra that can be fitted with the "log-parabola" (or log-normal) form. It might appear surprising that the sum of contributions with different spectral shapes generate a spectrum that has a simple power-law form. We suggest that this fact reveals some important properties for the ensemble of the extragalactic sources, and indicates that the blazar high energy emission can be considered as a "critical phenomenon". A curved log--parabola (log-normal) form is also required to fit the spectra of many Galactic gamma-ray sources, including the emission from the brightest supernova remnants (SNR), and an intriguing possibility is that also the Galactic cosmic rays are injected in interstellar space by their sources not with a single universal power-law spectrum, as predicted by the commonly accepted models, but instead with a broad range of spectral shapes.

中文翻译：

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河外伽马射线通量幂律形式的起源

Fermi-LAT 望远镜测量了所有河外源集合产生的河外伽马射线背景 (EGB)。EGB 的能量分布被很好地描述为具有大约等于 2.3 的光谱指数和指数截止的幂律，这与假设发射为不间断功率的高能光子的吸收效应一致。法谱。费米望远镜的观测也解决了 EGB，确定它的大部分是由耀变体类活动星系核 (AGN) 贡献的总和形成的。单个 AGN 源的光谱具有广泛的光谱形状，最亮和最精确测量的源具有可以拟合“对数抛物线”（或对数正态）形式的弯曲光谱。具有不同频谱形状的贡献的总和生成具有简单幂律形式的频谱，这似乎令人惊讶。我们认为这一事实揭示了河外源系综的一些重要特性，并表明耀变体高能发射可以被视为一种“临界现象”。还需要弯曲的对数抛物线（对数正态）形式来拟合许多银河伽马射线源的光谱，包括最亮的超新星遗迹 (SNR) 的发射，而且一个有趣的可能性是银河宇宙射线被它们的来源注入星际空间的不是像普遍接受的模型预测的那样具有单一的通用幂律谱，而是具有广泛的光谱形状。