[论文解读] The connection between radio and high energy emission in black hole powered systems in the SKA era

M. Giroletti, M. Orienti+10 more|arXiv (Cornell University)|Jan 14, 2015

Astrophysics and Cosmic Phenomena参考文献 3被引用 4

一句话总结

本文提出，平方公里阵列（SKA）将通过实现对高能源的高灵敏度、高分辨率射电观测，特别是对未识别的伽马射线源（UGS）的观测，解决耀变体中的射电-伽马射线关联问题。结合SKA与切伦科夫望远镜阵列（CTA）的甚高能（VHE）观测，将阐明伽马射线辐射的物理机制，检验粒子加速模型，并确定UGS是否为耀变体、暗物质候选者或奇特天体。

ABSTRACT

Strong evidence exists for a highly significant correlation between the radio flux density and gamma-ray energy flux in blazars revealed by Fermi. However, there are central issues that need to be clarified in this field: what are the counterparts of the about 30% of gamma-ray sources that are as yet unidentified? Are they just blazars in disguise or they are something more exotic, possibly associated with dark matter? How would they fit in the radio-gamma ray connection studied so far? With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all of these questions. Even more, while the radio-MeV/GeV connection has been firmly established, a radio-VHE connection has been entirely elusive so far. The advent of CTA in the next few years and the expected CTA-SKA1 synergy will offer the chance to explore this connection, even more intriguing as it involves the opposite ends of the electromagnetic spectrum and the acceleration of particles up to the highest energies. We are already preparing to address these questions by exploiting data from the various SKA pathfinders and precursors. We have obtained 18 cm European VLBI Network observations of E>10 GeV sources, with a detection rate of 83%. Moreover, we are cross correlating the Fermi catalogs with the MWA commissioning survey: when faint gamma-ray sources are considered, pure positional coincidence is not significant enough for selecting counterparts and we need an additional physical criterion to pinpoint the right object. It can be radio spectral index, variability, polarization, or compactness, needing high angular resolution in SKA1-MID; timing studies can also reveal pulsars, which are often found from dedicated searches of unidentified gamma-ray sources. SKA will be the ideal instrument for investigating these characteristics in conjunction with CTA. (abridged)

研究动机与目标

• 通过建立射电与高能辐射之间可靠的关联，阐明耀变体中伽马射线辐射的物理起源。
• 识别费米-LAT探测到的约30%未识别伽马射线源（UGS）的对应体，确定其是否为耀变体、暗物质候选者或奇特天体。
• 利用SKA与CTA的协同观测，探索射电-甚高能（VHE，>0.1 TeV）伽马射线关联的神秘关系。
• 利用SKA提供的高分辨率、偏振、多频射电数据，约束耀变体喷流中辐射区域的大小、磁场结构及变异性。
• 通过以亚毫毫ikel为敏感度、角秒级分辨率对南天进行巡天，实现对暗淡和高红移耀变体的统计研究。

提出的方法

• 利用SKA路径向探测器和先导项目（如EVN、MWA）的现有及未来射电数据，与费米-LAT星表进行交叉相关，实现源的关联。
• 利用高角分辨率SKA1-MID数据，应用多波段判据——谱指数、变异性、偏振度、紧凑性——识别UGS的真实对应体。
• 通过SKA1-MID与CTA的同步射电与伽马射线观测，减少因非同时观测导致的射电-伽马射线相关性中的离散度。
• 利用射电辐射的偏振与谱分析，探测伽马射线辐射区域的磁场强度与几何结构。
• 利用SKA1-MID的子阵列化与灵活调度，以极短观测时间实现对约2,000个耀变体的深度巡天，达到约30 μJy的极限。
• 通过SKA-MID波段5在>10 GHz的观测，将分析扩展至高红移源，以获取源系毫微微米波发射，探测喷流最内区。

实验结果

研究问题

• RQ1 费米-LAT探测到的约30%未识别伽马射线源的真实对应体是什么？它们是耀变体、暗物质信号，还是奇特源？
• RQ2 能否建立显著的射电与甚高能（VHE，>0.1 TeV）伽马射线发射之间的相关性？其结果揭示了何种粒子加速机制？
• RQ3 射电谱指数、偏振度与紧凑性如何帮助区分暗淡源群体中真实伽马射线对应体与位置巧合？
• RQ4 射电与伽马射线变异性中的非同时性在多大程度上导致了射电-伽马射线相关性中的观测离散度？
• RQ5 SKA1-MID与CTA能否联合实现对射电与伽马射线辐射区域的时间分辨、高分辨率研究，以约束辐射区域的大小与结构？

主要发现

• 在费米$E>10$ GeV源的18厘米欧洲甚长基线干涉网（EVN）观测中，检测率达到83%，未识别源的检测率更高（>50%）。
• 对于暗淡伽马射线源，仅靠位置巧合不足以实现关联；必须结合谱指数、变异性与偏振等物理判据，而这些判据依赖于SKA1-MID提供的高角分辨率。
• 即使仅使用全设计的50%，通过巧妙的调度与子阵列化，SKA1-MID在数小时内即可完成对所有可见费米AGN（包括最暗的耀变体，估计在2LAC中为~4.2 mJy）的巡天。
• 对最暗的约100个耀变体，可在不显著增加总观测时间的前提下，实现1%水平的偏振发射检测。
• 当前射电-伽马射线相关性存在显著离散度，限制了其在基于射电光度函数约束耀变体对河外伽马射线背景贡献时的使用。
• 完整的SKA将实现对单个耀变体在>10 GHz频率下的高分辨率、时间分辨、偏振观测，使高红移源（z ~ 5–8）的源系毫微微米发射得以访问，从而探测喷流最内区。