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

黑洞也许是宇宙中最奇特和神秘的物体。 根据爱因斯坦的广义相对论的预测，并被广泛接受为大质量单星和双星演化的某些例子的产物，最引人注目的例子是在几乎所有星系中心发现的“超大质量”黑洞。 在这里，吸积已经将200亿颗恒星中存在的物质聚集到了非常小的体积中。 我们的银河系是一个相对较小的例子，大约有400万个太阳质量的黑洞聚集在一个不比天王星绕太阳轨道大的区域内。到目前为止，黑洞的微小尺寸加上它们的巨大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（史密森天体物理天文台）和合作者正在采用射电天文学家开创的甚大基线干涉测量（VLBI）技术，将分散在地球周围的几个望远镜发出的联合收割机信号结合起来，目的是对来自黑洞不归点之外的无线电发射进行成像--也就是说，这个半径被称为史瓦西半径，在这个半径处，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。为了有效地将来自相距甚远的望远镜的信号联合收割机组合起来，每个信号都必须用非常精确的本地参考信号加时间戳。 该提案寻求资金，以获得超稳定的频率参考，用于全球VLBI观测毫米波长，旨在获得我们银河系中心超大质量黑洞的直接环境的前所未有的视图，也可能是潜伏在附近巨大星系M87中心的更大质量的洞。 幸运的是，这样的时间标准已经使用氢脉泽振荡器在大地测量和导航应用中开发出来，因此是合理负担得起的，而且非常可靠。 美国国家科学基金会天文科学部通过参与重大研究仪器计划，为全球VLBI网络的时间标准采购提供资金。

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.