[论文解读] Axion-like particles explain the unphysical redshift-dependence of AGN gamma-ray spectra

Giorgio Galanti, M. Roncadelli+2 more|arXiv (Cornell University)|Mar 15, 2015

Astrophysics and Cosmic Phenomena被引用 4

一句话总结

该论文提出轴子样粒子（ALPs）可解释在耀变体的甚高能（VHE）伽马射线谱中观测到的非物理红移依赖性。通过在星系际磁场中的光子-ALP振荡，观测到的谱指数变为与红移无关，从而解决了长期存在的不一致问题，并为ALPs及其在冷暗物质中的作用提供了间接证据。

ABSTRACT

Blazars are a class of AGN known to be powerful very-high-energy (VHE, 100 GeV - 100 TeV) celestial gamma-ray emitters. At the time of writing, 41 blazars, spread all over the sky and with known redshift in the range $0.0215 \leq z \leq 0.635$ have been observed in the VHE band by the Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC and VERITAS. Thus, they represent an isotropic and relatively local extragalactic sample, unaffected by significant cosmological evolution. The blazar emitted spectra are well fitted by a power law with index $\Gamma_{ m em}$. We show that the $\Gamma_{ m em}$ distribution exhibits an unexpected and previously unnoticed unphysical redshift-dependence. We demonstrate that this result is not due to any selection effect. It is difficult to imagine an intrinsic mechanism which could lead to such a spectral variation, and so this result seriously challenges the conventional view. We propose that such a behaviour is explained by oscillations between the VHE gamma-rays and Axion-Like Particles (ALPs), taking place in extragalactic magnetic fields. We recall that ALPs are predicted by several extensions of the Standard Model and especially by those based on superstring theories. Moreover, they are attracting growing interest being also good candidates for cold dark matter. As a consequence of the photon-ALP oscillation mechanism, the $\Gamma_{ m em}$ distribution becomes redshift-independent, indeed in agreement with the physical expectation. This is a highly nontrivial fact, which therefore provides a preliminary evidence for the existence of ALPs. Thus, besides physics laboratory data, astrophysical VHE data from e.g. the upcoming CTA can settle this issue. Our Universe may in this way be offering us a compelling reason to push physics beyond the Standard Model along a very specific direction and can shed light on the nature of cold dark matter.

研究动机与目标

• 解决耀变体VHE伽马射线谱指数的非物理红移依赖性问题，该问题与物理预期相矛盾。
• 研究这一异常是否可由标准模型之外的物理机制解释，特别是轴子样粒子（ALPs）。
• 检验星系际磁场中ALP诱导的光子-ALP振荡是否能将观测到的谱演化与宇宙学预期相协调。
• 提供一个物理机制，将红移引起的表观谱软化解释为真空极化与混合的产物，而非源本身的演化。
• 将VHE伽马射线观测定位为探测ALPs及冷暗物质候选者的工具。

提出的方法

• 分析由H.E.S.S.、MAGIC和VERITAS观测的41个红移在0.0215至0.635之间的耀变体的VHE伽马射线谱。
• 使用幂律模型拟合观测谱，以提取本征谱指数Γ_m_em。
• 识别出Γ_m_em中存在系统性、非物理的红移依赖性，且无法归因于选择效应。
• 使用混合角和ALP质量参数，建模星系际磁场中的光子-ALP振荡。
• 模拟光子与ALP之间能量依赖的转换概率，以及反向转换，从而修改观测谱。
• 将观测到的Γ_m_em红移演化与ALP振荡模型的预测进行比较，以检验一致性。

实验结果

研究问题

• RQ1 为何观测到的耀变体VHE伽马射线谱指数表现出与物理预期相反的系统性红移依赖性？
• RQ2 这种红移依赖性是否可能是真空极化或光子-ALP混合的产物，而非源本身的演化？
• RQ3 星系际磁场中轴子样粒子振荡在多大程度上可解释观测到的红移引起的谱软化？
• RQ4 在ALP模型下，有效谱指数的红移不变行为是否与观测数据一致？
• RQ5 VHE伽马射线观测能否作为探测ALPs存在及其在冷暗物质中作用的探针？

主要发现

• 在41个红移0.0215 ≤ z ≤ 0.635的耀变体中，观测到的VHE伽马射线谱指数（Γ_m_em）表现出显著的、非物理的红移依赖性。
• 该红移依赖性并非由观测选择效应引起，排除了仪器或样本偏差作为原因的可能性。
• 在星系际磁场中引入光子-ALP振荡后，Γ_m_em的红移依赖性被消除，其分布变为与红移无关，符合物理预期。
• 该模型自然地将红移引起的谱软化解释为光子-ALP混合的结果，而非源本身的演化。
• ALP模型与观测结果的一致性构成了轴子样粒子存在的初步、非平凡证据。
• 结果表明，未来望远镜如CTA的VHE伽马射线数据可对ALP假说进行决定性检验。