Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
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DOI：
10.1093/mnras/stab2406
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发表时间：
2021-01
影响因子：
4.8
通讯作者：
D. Song;O. Macias;S. Horiuchi;R. Crocker;D. Nataf
中科院分区：
文献类型：
--
作者：
D. Song;O. Macias;S. Horiuchi;R. Crocker;D. Nataf
毫秒双星很可能是球状星团伽马射线辐射的主要来源。然而，两个独立的发射过程的相对贡献-毫秒脉冲星磁层的曲率辐射与毫秒脉冲星发射到球状星团环境中的相对论对的逆康普顿发射-长期以来一直不清楚。为了解决这个问题，我们寻找逆康普顿发射的证据，在8年的费米-LAT数据从157个银河系球状星团的方向。我们发现球状星团γ射线的光度与其光子场能量密度之间存在着轻微的统计显著相关性（3.8σ）。然而，这也可以解释的光子场密度和球状星团的恒星相遇率之间的隐藏的相关性。全面分析，我们证明了球状星团的γ射线辐射可以分解成两个组成部分：（i）指数截止幂律和（ii）纯幂律。后一个分量--我们发现其显著性为8.2σ --的幂指数为2.79 ± 0.25。它最自然地被解释为毫秒脉冲星注入的宇宙射线电子和正电子的逆康普顿发射。我们发现这个幂律分量的光度与弯曲分量的光度相当，或略小于弯曲分量的光度，这表明毫秒脉冲星自旋向下光度到相对论轻子的分数与自旋向下光度到瞬发磁层辐射的分数相似。
Millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. However, the relative contributions of two separate emission processes – curvature radiation from millisecond pulsar magnetospheres versus inverse Compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars – have long been unclear. To address this, we search for evidence of inverse Compton emission in 8-yr Fermi–LAT data from the directions of 157 Milky Way globular clusters. We find a mildly statistically significant (3.8σ) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. However, this may also be explained by a hidden correlation between the photon field densities and the stellar encounter rates of globular clusters. Analysed in toto, we demonstrate that the gamma-ray emission of globular clusters can be resolved spectrally into two components: (i) an exponentially cut-off power law and (ii) a pure power law. The latter component – which we uncover at a significance of 8.2σ – has a power index of 2.79 ± 0.25. It is most naturally interpreted as inverse Compton emission by cosmic-ray electrons and positrons injected by millisecond pulsars. We find the luminosity of this power-law component is comparable to, or slightly smaller than, the luminosity of the curved component, suggesting the fraction of millisecond pulsar spin-down luminosity into relativistic leptons is similar to the fraction of the spin-down luminosity into prompt magnetospheric radiation.