Exploring the Unknown Very High Energy Gamma-Ray Sky

高能伽马射线被用来研究宇宙中最极端的物体，如遥远星系中心的黑洞和伽马射线爆发，这是自大爆炸以来最强大的爆炸。宇宙伽马射线还可以提供新的物理原理的线索，超出了目前对亚原子物理和宇宙学的理解范围。这项研究中研究的超高能(VHE)伽马射线的能量大约是用于医学图像的X射线的1亿倍。然而，它们很难被探测到，因为它们会被大气吸收。在较低的能量下，来自天体物理源的伽马射线的数量足以用大气层上方轨道上的卫星上的探测器进行观测。最高能量的伽马射线要罕见得多，需要有足球场大小的探测器，这样的探测器只能建在地面上。它们通过探测伽马射线与大气中空气分子碰撞时产生的次级粒子簇射来间接观测伽马射线。研究人员将在高海拔地区使用一个大型的仪器储水池--名为“Milagro”--来探测到达地面的伽马射线阵雨中的次级粒子。这座人造水库的蓄水量约为500万加仑。这些颗粒在水中产生闪光，由称为光电倍增管的灵敏探测器记录下来。水库上方不透明的黑色盖子挡住了阳光，这样就可以检测到微弱的闪光，水也经过了过滤，所以非常清澈。米拉格罗对主要能量在1000亿电子伏特到10万亿电子伏特之间的伽马射线很敏感(可见光的能量只有几个电子伏特)。Milagro是由研究人员及其合作者专门为此目的而建造的，主要是在NSF的支持下建造的。这项提议的重点是利用Milagro通过三个主要的科学主题来探索宇宙中未知的VHE行为：伽马射线爆发的发射，测量天空，以及更深入地研究已知的但未确定的来源。伽马射线暴是宇宙中已知的最强大的爆炸，但它们是如何工作的细节，以及它们在其他高能现象中扮演的角色(例如，它们是最高能量宇宙射线的加速器吗？)仍在探索中。了解它们是否能够在一般情况下、在特定情况下或根本不能产生VHE伽马射线，将限制伽马射线暴的模型和爆发环境的性质。观测天空和研究不明来源是齐头并进的。调查是揭示新来源的最好方法，不受任何关于VHE排放的先入为主的观念的影响。其中一些新来源将与在其他波长(例如无线电波、光学或X射线)探测到的已知物体有明确的关联，并可能会也可能不会在其VHE发射方面产生惊喜。其他的，不明来源，拥有发现新物体和新现象的最大潜力。这项提案的工作将有助于提高公众的科学素养，促进理科学生的教育发展，以及国家的技术基础设施。公众对宇宙的好奇心创造了许多机会来激发他们的兴趣，对于学生来说，也创造了参与的机会。博士后、研究生和本科生，包括许多来自代表性不足的群体，在研究工作中发挥了重要作用。研究人员定期与K-12年级的学生互动，他们既是学校的访问者，也是大学实验室的东道主，并向公众介绍普遍感兴趣的讲座。与洛斯阿拉莫斯国家实验室在米拉格罗项目上的合作，通过帮助吸引和留住那里的顶尖人才，推动了该实验室的国家安全使命。

High-energy gamma rays are used to study the most extreme objects in the Universe, such as the black holes at the centers of distant galaxies and gamma-ray bursts, the most powerful explosions since the Big Bang. Cosmic gamma rays can also provide clues to new physical principles beyond the scope of the current understanding of subatomic physics and cosmology. The very high-energy (VHE) gamma rays studied in this research have about 100 million times as much energy as the X-rays used for medical images. None the less, their detection is difficult because they are absorbed by the atmosphere. At the lower energies, the number of gamma rays from astrophysical sources is sufficient to observe with detectors on board satellites in orbit above the atmosphere. The highest energy gamma rays are much rarer and require detectors with an area the size of a soccer field, Such detectors can only be built on the ground. These observe gamma rays indirectly by detecting the shower of secondary particles produced when a gamma ray collides with an air molecule in the atmosphere. The researchers will use a large, instrumented reservoir of water at high altitude-called "Milagro" -to detect the secondary particles from gamma-ray showers reaching the ground. The man-made reservoir holds about 5 million gallons of water. The particles produce flashes of light in the water, which are recorded by sensitive detectors called photomultiplier tubes. An opaque black cover over the reservoir keeps out sunlight so that the faint flashes can be detected, and the water is filtered so that it is very clear. Milagro is sensitive to gamma rays primarily with energy between about 100 billion electron-volts and 10 trillion electron-volts (visible light has energies of a few electron-volts). Milagro was built specifically for this purpose by the researchers and their collaborators, primarily with support from NSF. This proposal focuses on using Milagro to explore the unknown VHE behavior of the Universe by way of three principal science topics: emission from gamma-ray bursts, surveying the sky, and studying in more depth the known, but unidentified, sources. Gamma-ray bursts are the most powerful explosions known in the Universe, but the details of how they work and the role they play in other high energy phenomena (For example, are they the accelerators of the highest energy cosmic rays?) are still being explored. Learning whether they are capable of creating VHE gamma rays in general, in particular circumstances, or not at all, will constrain gamma-ray burst models and properties of the burst environment. Surveying the sky and studying the unidentified sources go hand in hand. Surveys are the best way to reveal new sources independent of any preconceived notions about VHE emission. Some of these new sources will have clear associations with known objects detected at other wavelengths (for example, radiowaves, optical light, or X-rays) and may or may not produce surprises in terms of their VHE emission. Others, the unidentified sources, hold the greatest potential for the discovery of new objects and phenomena. The work of this proposal will contribute to the scientific literacy of the public, the educational development of students in the sciences, and the nation's technological infrastructure. The public's curiosity about the Universe creates many opportunities to engage their interest and, in the case of students, participation. Postdoctoral, graduate and undergraduate students, including many from underrepresented groups, play an important part in the research effort. The researchers interact regularly with K-12 students, both as visitors to schools and hosts in their university labs as well as presenting general interest lectures to the public. The collaboration with Los Alamos National Laboratory on the Milagro project advances the national security mission of the Lab by helping to attract and retain top-notch talent there.