MRI: Acquisition of Stable Hydrogen Maser Frequency Standards for Millimeter/Submillimeter VLBI Observations of a Black Hole Event Horizon

黑洞可能是宇宙中最奇异、最神秘的物体。爱因斯坦的广义相对论预言了这一点，并被广泛认为是某些大质量单星和双星演化的产物，最引人注目的例子是在几乎所有星系的中心发现的“超大质量”黑洞。在这里，吸积将200亿颗恒星中存在的物质聚集到非常小的体积中。我们的银河系是一个相对较小的例子，在一个不比天王星绕太阳轨道大的区域内，大约有400万个太阳质量。到目前为止，黑洞的微小尺寸加上它们遥远的距离使我们只能看到黑洞对周围恒星和气体的引力效应。然而，S. Doeleman博士（史密森天体物理天文台）和合作者正在使用射电天文学家首创的甚大基线干涉测量（VLBI）技术，将分散在地球周围的几架望远镜的信号结合起来，目的是成像黑洞不返回点外的射电发射——所谓的史瓦西半径，在这里流入黑洞的物质发出的最后一束光可以逃逸并到达外部观察者。为了有效地结合来自相隔很远的望远镜的信号，每个信号都必须加上一个极其精确的本地参考信号的时间戳。该提案寻求资金，以获得全球超大质量黑洞观测的超稳定频率参考，以获得银河系中心超大质量黑洞周围的前所未有的视野，也可能是潜伏在附近巨大星系M87中心的更大质量的黑洞。幸运的是，这种时间标准是使用氢脉泽振荡器开发的，用于大地测量学和导航，因此价格合理且非常可靠。美国国家科学基金会天文科学部通过参与主要研究仪器计划，为全球VLBI网络的时间标准采集提供资金。

Black holes are perhaps the most exotic and enigmatic objects in the universe. Predicted by Einstein's theory of General Relativity and widely accepted as products of certain examples of massive single-star and binary-star evolution, the most dramatic cases are the "supermassive" black holes found in the centers of essentially all galaxies. Here, accretion has amassed as much matter as exists in 20 billion stars into extraordinarily tiny volumes. Our Galaxy hosts a comparatively modest example, with some 4 million solar masses packed inside a region no larger than Uranus' orbit around the Sun.Until now, the tiny sizes of black holes coupled with their vast distances have permitted us to view only the gravitational effects of black holes on surrounding stars and gas. However, Dr. S. Doeleman (Smithsonian Astrophysical Observatory) and collaborators are employing the Very Large Baseline Interferometry (VLBI) technique pioneered by radio astronomers to combine signals from several telescopes scattered around the Earth with the goal of imaging the radio emission from just outside a black hole's point of no return - the so-called Schwarzschild radius where the last light emitted by matter flowing into the hole can escape and reach an outside observer.To effectively combine the signals from widely separated telescopes, each signal must be time-stamped with an extremely precise local reference signal. This proposal seeks funds to acquire ultra-stable frequency references for global VLBI observations at mm-wavelengths aimed at obtaining unprecedented views of the immediate environs of the supermassive black hole at the center of our Milky Way, and possibly also of the much more massive hole that lurks at the center of the nearby giant galaxy M87. Fortunately, such time standards have been developed using hydrogen maser oscillators for applications in geodesy and navigation, and therefore are reasonably affordable and extremely reliable. Funding for the acquisition of time standards for the global VLBI network is being provided by NSF's Division of Astronomical Sciences through its participation in the Major Research Instrumentation program.