Operation of the Milagro Gamma Ray Observatory

超高能伽马射线天文学探测已知宇宙中一些最极端的环境。已知的TeV伽马射线源包含具有强烈引力场和/或磁场的致密天体(黑洞或中子星)。已知的TeV伽马射线源包括银河系内的超新星遗迹和银河系外的活动星系核。其他可能的来源包括伽马射线爆发、微类星体、宇宙射线与我们银河系内物质相互作用的漫射辐射，以及更奇异的物体，如原始黑洞和弱相互作用的大质量粒子。在北半球已经探测到六个TeV光子源。已知源的缺乏既是由于现有的仪器设备，也是由于源。瞬变震源和窄场仪器的结合使新震源的探测变得困难。Milagro是第一个能够在几百GeV以上的能量下连续监测整个头顶天空的探测器。Milagro是一个广泛的水切伦科夫空气簇射探测器，位于新墨西哥州洛斯阿拉莫斯附近，海拔2630米，由一个约5,000平方米的中央(池塘)探测器组成，周围环绕着175个仪表式水箱(外伸支腿)，面积约为40,000平方米。米拉格罗池塘是一个600万加仑的蓄水池，深80米x 60米x 8米，覆盖着不透光的盖子。该水库在2.8m x 2.8m的网格上安装了723个20厘米的PMT。PMT部署在两个层中。顶层450艘PMTS潜水1.5米，底层273艘PMTS潜水6米。簇射粒子到达的时间被用来确定主风簇射的方向。在我们运行的三年中，我们开发了一种可靠的方法来减少宇宙线本底，并使用这种拒绝方法来探测蟹状星云MRK 421的TeV伽马射线，并看到了银河系平面弥漫TeV辐射的第一个证据。此外，我们还监测了北半球的瞬变和稳定TeV光子源。到目前为止，我们只观察到上述三个来源的排放。我们现在已经为北半球的TeV源设定了从1毫秒到2.5年的所有时间尺度的最佳限制。在过去的一年里，我们实施了一个实时分析系统，使我们能够在伽马射线暴发生后4秒内探测到它，并向其他天文台发出警报。在1毫秒到2小时的时间尺度上，我们还没有从我们视野内的任何区域探测到显著的辐射。在此期间，HETE和积分卫星探测到的爆发相对较少。今年晚些时候，SWIFT的发射将为我们提供丰富的数据集，以搜索伽马射线暴的重合TeV发射。正如我们的原型Milagrito观测到了由BASTE探测到的一个伽玛暴的显著排放一样，我们预计SWIFT的出现将证明Milagro将是一个富有成果的时期。这项提议将允许我们在未来三年内继续运营Milagro。在过去的两年里，对探测器进行了几次改进。我们已经实施了一种智能触发系统，使我们能够大幅降低Milagro的触发阈值。这使得100GeV的有效面积增加了四倍--这是探测伽马射线爆发的关键。最重要的是，我们刚刚完成了围绕池塘的175个水箱的建设。这些坦克提高了能量和角度分辨率以及米拉格罗的背景抑制能力。我们已经获得了3倍的灵敏度增加，因为这些探测器被纳入我们的事件重建。米拉格罗数据很容易向学生和公众解释，他们通常很难理解我们是如何声称看到并不存在的粒子的。这次合作非常活跃，让学生和普通公众接触到粒子天体物理学(PA)领域的兴奋。每年我们都会带本科生来研究米拉格罗，通常让学生参与到分析中来。此外，我们每年平均在高中举办八次讲座，以及至少两次以PA为主题的公开讲座。他还与美国物理奥林匹克代表队合作。此外，他还参与了马里兰州8年级和10年级女孩的项目。他还发起了世界各地许多教师都在使用的物理“一周问题”(请访问www.Physiics.umd.edu)。

Very high energy gamma ray astronomy probes some of the most extreme environments in the known universe. Known sources of TeV gamma rays contain compact objects (black holes or neutron stars) with intense gravitational and/or magnetic fields. Known sources of TeV gamma rays include supernova remnants within our galaxy and active galactic nuclei outside of our galaxy. Other possible sources include gamma-ray bursts, micro-quasars, diffuse emission from cosmic rays interacting with matter in our own galaxy, and more exotic objects such as primordial black holes and weakly interacting massive particles. Six sources of TeV photons have been detected in the northern hemisphere. The paucity of known sources is due to both the available instrumentation and the sources. The combination of transient sources and narrow field instruments makes the detection of new sources difficult. Milagro is the first detector capable of continuously monitoring the entire overhead sky at energies above a few hundred GeV. Milagro is a water Cherenkov extensive air shower detector located near Los Alamos, NM at 2630m above sea level, consisting of a ~5,000 m2 central (pond) detector surrounded by an array of 175 instrumented water tanks, (outriggers) that span an area of roughly 40,000 m2. The Milagro pond is a 6-million gallon water reservoir, which measures 80m x 60m x 8m deep and is covered with a light-tight cover. The reservoir is instrumented with 723 20-cm PMTs on a 2.8m x 2.8m grid. The PMTs are deployed in two layers. The top layer of 450 PMTs is under 1.5 meters of water and the bottom layer of 273 PMTs is under 6 meters of water. Timing of the arrival of the shower particles is used to determine the direction of the primary air shower. During the three years we have been operating we have developed a robust method for reducing the cosmic ray background and used this rejection method to detect TeV gamma rays from the Crab Nebula, Mrk 421, and to see the first evidence for diffuse TeV emission from the galactic plane. In addition, we have monitored the northern hemisphere for both transient and steady sources of TeV photons. To date we have observed emission only from the three sources mentioned above. We have now set the best limits on TeV sources in the northern hemisphere over all timescales from 1 millisecond to 2.5 years. During the past year we have implemented a real-time analysis system that allows us to detect a gamma-ray burst within 4 seconds of its occurrence and to send an alert to other observatories. We have not yet detected significant emission from any region within our field of view on timescales from 1ms to 2hrs. During this period there were relatively few bursts detected by the HETE and INTEGRAL satellites. Later this year the launch of SWIFT will provide us with a rich dataset to search for coincident TeV emission from gamma-ray bursts. Just as Milagrito, our prototype, observed significant emission from one GRB detected by BASTE we expect the advent of SWIFT to prove a fruitful period for Milagro. This proposal will allow us to continue to operate Milagro for the next three years. During the past two years several improvements have been made to the detector. We have implemented an intelligent triggering system that has enabled us to substantially lower the trigger threshold of Milagro. This has increased the effective area at 100 GeV four-fold - critical for detecting gamma-ray bursts. Most importantly, we have just completed the construction of an array of 175 water tanks that surround the pond. These tanks improve the energy and angular resolution and the background rejection capabilities of Milagro. We have obtained a 3-fold increase in sensitivity now that these detectors are incorporated into our event reconstruction. Milagro data are easy to explain to students and the general public, who often have difficulty understanding how we claim to see particles that they don't. The collaboration has been very active in exposing students and the general public to the excitement of the particle astrophysics (PA) field. Each year we bring undergraduate students to work on Milagro and generally involve students in analysis. In addition, we give on average eight talks each year at high schools and at least two public lectures on the subject of PA. He also works with the US Physics Olympics team. In addition, he works with the Maryland program for 8th and 10th grade girls. He also started the physics "Question of the Week" (see it at www.physics.umd.edu) that is used by many teachers world-wide.