[论文解读] Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter

Dan Hooper, Tracy R. Slatyer|arXiv (Cornell University)|Feb 26, 2013

Dark Matter and Cosmic Phenomena被引用 18

一句话总结

该论文提出，低纬度费米泡的伽马射线谱在1–4 GeV处表现出一个与宇宙射线电子逆康普顿散射不一致的显著特征，归因于第二种辐射机制。该成分在能谱和空间分布上与10 GeV暗物质湮灭至轻子或50 GeV湮灭至夸克的信号一致，且其分布比NFW更陡，为银河系中心过量的延伸部分提供了有力的暗物质解释。

ABSTRACT

We study the variation of the spectrum of the Fermi Bubbles with Galactic latitude. Far from the Galactic plane (|b| > 30 degrees), the observed gamma-ray emission is nearly invariant with latitude, and is consistent with arising from inverse Compton scattering of the interstellar radiation field by cosmic-ray electrons with an approximately power-law spectrum. The same electrons in the presence of microgauss-scale magnetic fields can also generate the the observed microwave "haze". At lower latitudes (b < 20 degrees), in contrast, the spectrum of the emission correlated with the Bubbles possesses a pronounced spectral feature peaking at 1-4 GeV (in E^2 dN/dE) which cannot be generated by any realistic spectrum of electrons. Instead, we conclude that a second (non-inverse-Compton) emission mechanism must be responsible for the bulk of the low-energy, low-latitude emission. This second component is spectrally similar to the excess GeV emission previously reported from the Galactic Center (GC), and also appears spatially consistent with a luminosity per volume falling approximately as r^-2.4, where r is the distance from the GC. We argue that the spectral feature visible in the low-latitude Bubbles is the extended counterpart of the GC excess, now detected out to at least 2-3 kpc from the GC. The spectrum and angular distribution of the signal is consistent with that predicted from ~10 GeV dark matter particles annihilating to leptons, or from ~50 GeV dark matter particles annihilating to quarks, following a distribution similar to the canonical Navarro-Frenk-White (NFW) profile. We also consider millisecond pulsars as a possible astrophysical explanation for the signal, as observed millisecond pulsars possess a spectral cutoff at approximately the required energy. Any such scenario would require a large population of unresolved millisecond pulsars extending at least 2-3 kpc from the GC.

研究动机与目标

• 研究费米泡在不同银纬下的能谱变化，以识别与标准逆康普顿辐射的偏离。
• 确定低银纬处观测到的伽马射线谱是否可由单一电子种群解释，或是否需要第二种辐射机制。
• 检验1–4 GeV处的低能谱特征是否与暗物质湮灭一致，特别是与银河系中心过量的对比。
• 评估其他天体物理解释（如未解析的毫秒脉冲星）的可行性。

提出的方法

• 对费米-LAT数据在不同银纬下进行各向异性的谱分析，重点关注|b| > 30°和|b| < 20°区域。
• 建模幂律电子种群（dNe/dE ∝ E−3）与星际辐射场相互作用产生的逆康普顿散射。
• 在微高斯量级磁场下估算相同电子种群产生的同步辐射，以匹配射电晕特征。
• 通过假设暗物质湮灭起源，对低银纬伽马射线谱进行拟合，包含特定末态和质量。
• 通过比较空间形态（单位体积亮度 ∝ r−2.4）与NFW及更陡的分布，评估暗物质分布的兼容性。
• 通过要求在银河系中心附近2–3 kpc范围内存在密集且未解析的脉冲星种群，评估毫秒脉冲星情景。

实验结果

研究问题

• RQ1 高银纬处（|b| > 30°）近乎平坦、硬谱的伽马射线发射是否可由单一电子种群的逆康普顿散射解释？
• RQ2 为何低银纬处（|b| < 20°）的伽马射线谱在1–4 GeV处表现出一个无法用逆康普顿过程解释的显著峰？
• RQ3 低银纬辐射的能谱和空间形态是否与暗物质湮灭一致，特别是与银河系中心过量的延伸对应部分？
• RQ4 毫秒脉冲星是否可能产生观测到的信号，以及需要多高的种群密度才能解释该发射？
• RQ5 观测到的亮度分布（r−2.4）与NFW等理论暗物质分布相比如何？

主要发现

• 高银纬费米泡（|b| > 30°）表现出近乎不变的平坦伽马射线谱，与幂律电子种群（dNe/dE ∝ E−3）的逆康普顿散射一致。
• 相同电子种群在微高斯量级磁场下自然产生与观测射电晕匹配的同步辐射，支持其共同起源。
• 在低银纬（|b| < 20°）区域，E²dN/dE在1–4 GeV处出现一个无法用逆康普顿散射解释的显著谱特征，表明存在第二种辐射机制。
• 该低银纬信号在空间上与单位体积亮度随r−2.4衰减的分布一致，与延伸的银河系中心过量匹配，暗示共同起源。
• 谱特征与形态总体与10 GeV暗物质粒子湮灭至轻子或50 GeV粒子湮灭至夸克的模型一致，且其分布比标准NFW剖面更陡。
• 若采用毫秒脉冲星解释，则需假设在银河系中心附近2–3 kpc范围内存在不切实际的高密度未解析脉冲星种群。