Particle Astrophysics with Milagro

粒子天体物理学与 Milagro

• 批准号: 0504201
• 负责人: Peter Nemethy
• 金额: $ 45万
• 依托单位: New York University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-11-01 至 2009-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0504201&HistoricalAwards=false
• 关键词: Particle; Astrophysics; Milagro

The Milagro Gamma Ray Observatory at Fenton Hill, New Mexico, is designed to study "Very High Energy" (VHE) astronomical gamma rays and cosmic rays, for which a single particle carries about an erg of energy. Milagro is the first detector capable of continuously monitoring the full overhead sky at these energies. It complements the lower energy satellite experiments, higher energy air-shower experiments, and narrow field-of-view air-Cherenkov telescope experiments. A VHE gamma ray or cosmic ray interacting in the Earth's atmosphere initiates an extensive air shower, a cascade of interacting particles. The Milagro 22 million liter man-made pond is instrumented with detectors that measure light produced by those fast moving cascade particles that survive to detector altitude and enter the pond. Combining pond measurements with those by an array of detectors surrounding it allows determination of the energy and arrival direction of the primary particle that generated the cascade. Since the energy of particles emitted by astronomical sources is determined by the physical processes occurring in the source, measurements at VHE energies, the highest at which astronomical sources have been observed, allows study of the most violent astronomical processes known. An example is gamma-ray bursts, currently believed to arise from either supernovae explosions or neutron star collisions. Active galactic nuclei, understood to contain massive black holes that swallow neighboring matter, are a detected source of VHE particles. Milagro has recently presented evidence for the first detection of VHE gamma rays emitted at the equator of our own Milky Way Galaxy. Milagro also detects solar coronal mass-ejection events, as an increase in detector rates, before they disrupt radio communication on Earth. The particle energy and direction distribution generated by a source may be modified during the particles' interstellar or intergalactic path to Earth by interactions with background particles or bending of charged particle trajectories by magnetic fields. Measurement of VHE particles therefore also teaches us about the interstellar infrared radiation and sources of magnetic fields such as galactic clusters. The Solar magnetic field is studied by observing the shadowing of arriving VHE cosmic rays by the Moon and Sun. Due to randomization of their arrival direction by galactic magnetic fields, cosmic rays arrive at the Earth nearly uniformly in direction. At VHE energies, however, small deviations are predicted due to the orbital motion of the solar system about the galactic center. Anisotropies observed by Milagro at the level of one part in a thousand are under study. Searches for exotic VHE photon sources, such as weakly interacting massive particle (WIMP) annihilation or evaporation of primordial black holes, are also performed. As VHE sources without known lower energy counterparts have just begun to be discovered in the last few years, full-sky coverage is likely to provide additional surprises. Cooperation between Milagro and various experiments at other energies allows symbiotic studies. The statistical power of the Milagro gamma ray burst analysis is enhanced when satellite detections are used as a trigger. Milagro will in turn be notifying the astrophysics community in real-time of any statistically significant signals it detects. Combined measurements by various experiments at different energies constrain models of particle production and interstellar background light absorption. Similar advantages exist when Milagro measurements of coronal solar mass-ejection events are combined with those of neutron monitors.

位于新墨西哥州芬顿山的米拉格罗伽马射线天文台旨在研究“极高能量”（VHE）天文伽马射线和宇宙射线，其中单个粒子携带着 erg 的能量。 Milagro 是第一个能够以这些能量连续监测整个头顶天空的探测器。它补充了低能量卫星实验、高能量空气簇射实验和窄视场切伦科夫空气望远镜实验。 VHE 伽马射线或宇宙射线在地球大气层中相互作用，引发了一场广泛的空气簇射，即一系列相互作用的粒子。 Milagro 2200 万升人造池塘配备了探测器，用于测量那些快速移动的级联粒子产生的光，这些粒子存活到探测器高度并进入池塘。将池塘测量值与周围探测器阵列的测量值相结合，可以确定产生级联的初级粒子的能量和到达方向。由于天文源发射的粒子能量是由源中发生的物理过程决定的，因此在 VHE 能量（已观测到的天文源的最高能量）下进行测量，可以研究已知的最剧烈的天文过程。一个例子是伽马射线暴，目前认为是由超新星爆炸或中子星碰撞引起的。据了解，活动星系核含有吞噬邻近物质的大质量黑洞，是检测到的 VHE 粒子来源。 Milagro 最近提出了首次探测到我们银河系赤道处发射的 VHE 伽马射线的证据。米拉格罗还可以在太阳日冕物质抛射事件扰乱地球上的无线电通信之前，通过增加探测器速率来探测它们。由源产生的粒子能量和方向分布可以在粒子到达地球的星际或星系间路径期间通过与背景粒子的相互作用或磁场引起的带电粒子轨迹的弯曲而被修改。因此，VHE 粒子的测量还让我们了解星际红外辐射和磁场源（例如星系团）。通过观察月球和太阳到达的 VHE 宇宙射线的阴影来研究太阳磁场。由于银河磁场的到达方向随机化，宇宙射线到达地球的方向几乎一致。然而，在 VHE 能量下，由于太阳系绕银河中心的轨道运动，预测会出现小偏差。米拉格罗观察到的千分之一水平的各向异性正在研究中。还对奇异的 VHE 光子源进行了搜索，例如弱相互作用大质量粒子 (WIMP) 湮灭或原始黑洞的蒸发。由于过去几年才开始发现没有已知较低能量对应物的 VHE 源，全天空覆盖可能会带来额外的惊喜。 Milagro 与其他能量的各种实验之间的合作允许进行共生研究。当使用卫星探测作为触发时，Milagro 伽马射线爆发分析的统计能力得到增强。 Milagro 将实时通知天体物理学界其检测到的任何具有统计意义的信号。在不同能量下进行的各种实验的综合测量限制了粒子产生和星际背景光吸收的模型。当米拉格罗对日冕太阳物质抛射事件的测量与中子监测仪的测量相结合时，也存在类似的优势。