Particle Astrophysics with Milagro

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“超高能”(VHE)天文伽马射线和宇宙射线，对于这些射线，单个粒子携带大约一erg的能量。米拉格罗是第一个能够在这些能量下连续监测整个头顶天空的探测器。它是对低能卫星实验、高能空气簇射实验和窄视场空气切伦科夫望远镜实验的补充。VHE伽马射线或宇宙射线在地球大气层中相互作用，会引发大范围的空气簇射，即相互作用的粒子的级联。在米拉格罗2200万升的人造池塘上安装了探测器，这些探测器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。将池塘的测量结果与周围探测器阵列的测量结果相结合，可以确定产生级联的初级粒子的能量和到达方向。由于天文震源发射的粒子能量是由震源中发生的物理过程决定的，因此对观测到的天文震源的最高能量进行测量，可以研究已知的最剧烈的天文过程。伽马射线爆发就是一个例子，目前人们认为它要么来自超新星爆炸，要么来自中子星碰撞。据了解，活动星系核含有吞噬邻近物质的大质量黑洞，是探测到的VHE粒子的来源。米拉格罗最近提出了首次探测到在我们银河系赤道发射的VHe伽马射线的证据。米拉格罗还探测到日冕物质抛射事件，在它们扰乱地球上的无线电通信之前，随着探测器速率的增加。在粒子到地球的星际或星际路径中，通过与背景粒子的相互作用或磁场弯曲带电粒子的轨迹，源产生的粒子能量和方向分布可能会被改变。因此，对VHe粒子的测量也让我们了解了星际红外辐射和磁场的来源，如星系团。太阳磁场是通过观测月亮和太阳对到达的宇宙射线的阴影来研究的。由于星系磁场对宇宙射线到达方向的随机化，宇宙射线到达地球的方向几乎是一致的。然而，在VHE能量下，由于太阳系围绕银河系中心的轨道运动，预测的偏差很小。米拉格罗观察到的千分之一的各向异性正在研究中。对奇异的VHE光子源的搜索也被执行，例如原始黑洞的弱相互作用大质量粒子(WIMP)湮灭或蒸发。由于过去几年才刚刚开始发现没有已知较低能量对应的VHE源，全天空覆盖可能会带来额外的惊喜。米拉格罗与其他能源的各种实验之间的合作使共生研究成为可能。当卫星探测被用作触发器时，Milagro伽马射线暴分析的统计能力得到了增强。反过来，米拉格罗将向天体物理学团体实时通知它检测到的任何统计上有意义的信号。各种实验在不同能量下的联合测量约束了粒子产生和星际背景光吸收的模型。当太阳日冕物质抛射事件的Milagro测量与中子监测器的测量相结合时，也存在类似的优势。

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.