Exploring the Unknown Very High Energy Gamma-Ray Sky

高能伽马射线用于研究宇宙中最极端的物体，例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学和宇宙学的理解范围。在这项研究中研究的极高能（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。尽管如此，由于它们被大气层吸收，所以很难探测到。在较低的能量下，来自天体物理源的伽马射线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，需要面积相当于足球场大小的探测器，这种探测器只能建在地面上。这些望远镜通过探测伽马射线与大气中的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。研究人员将在高空使用一个大型的、装有仪器的蓄水池--称为“米拉格罗”--来探测到达地面的伽马射线阵雨中的次级粒子。这座人造水库可容纳约500万加仑的水。这些粒子在水中产生闪光，这些闪光被称为光电倍增管的敏感探测器记录下来。蓄水池上有一个不透明的黑色盖子，可以挡住阳光，这样微弱的闪光就可以被探测到，水经过过滤，所以很清澈。米拉格罗对伽马射线敏感，主要是能量在1000亿电子伏特到10万亿电子伏特之间（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，主要得到了NSF的支持。该提案的重点是使用米拉格罗通过三个主要的科学主题来探索宇宙未知的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但身份不明的来源。伽马射线暴是宇宙中已知的最强大的爆炸，但它们如何工作的细节以及它们在其他高能现象中所扮演的角色（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够产生VHE伽马射线，将约束伽马射线暴模型和暴环境的属性。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佳方式，不受任何关于VHE排放的先入为主的观念的影响。其中一些新的来源将与在其他波长（例如，无线电波，可见光或X射线）检测到的已知物体有明确的联系，并且可能会或可能不会在VHE发射方面产生惊喜。其他的，未被确认的来源，拥有发现新物体和现象的最大潜力。这项提案的工作将有助于公众的科学素养，学生在科学方面的教育发展，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吸引他们的兴趣，并在学生的情况下，参与。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学生互动，无论是作为学校的访客还是大学实验室的主持人，以及向公众展示一般兴趣讲座。与洛斯阿拉莫斯国家实验室在Milagro项目上的合作通过帮助吸引和留住顶尖人才来推进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.