Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
Nature. 2011 Dec 14;481(7379):51-4. doi: 10.1038/nature10652.
Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.
恒星轨道的测量结果为一个强有力的证据，证明银河系中心的紧凑射电源 Sagittarius A* 是一个质量为太阳的四百万倍的黑洞。除了适度的 X 射线和红外耀斑之外，Sgr A* 出奇地暗淡，这表明在事件视界附近的吸积率和辐射效率目前非常低。在这里，我们报告了一个大约三倍于地球质量的密集气体云正在落入 Sgr A的吸积区。我们的观测结果严格限制了该云的轨道是高度偏心的，其最接近的内半径仅为事件视界的约 3100 倍，这将在 2013 年达到。在过去的三年中，该云已经开始解体，可能主要是通过黑洞引力产生的潮汐剪切。在接下来的几年中，云的动态演化和辐射将探测吸积流的性质和超大质量黑洞的馈送过程。当云到达近日点时，Sgr A 的千电子伏特 X 射线发射可能会显著变亮。如果云分裂并将其碎片送入中央吸积区，可能在几年后会有一个巨大的辐射耀斑。
