Eclipsing a black hole

A chance eclipse has enabled astronomers for the first time to measure the width of a disk of swirling, hot matter around a supermassive black hole.

BLOCKING BLACKNESS. These X rays come from the supermassive black hole at the core of the spiral galaxy NGC 1365. Last year, a cloud obscured them. CXC, NASA

The black hole lies at the center of the galaxy NGC 1365, some 60 million light-years from Earth. Scientists have proposed that gas and dust surrounding such a hole don’t simply fall in but form a rotating disk. Matter dragged inward from the disk would be heated to millions of kelvins and emit X rays before disappearing into the hole.

NASA’s Chandra X-ray Observatory has detected X rays from the core of NGC 1365, but the proposed disk would be too small for the craft to discern directly. For several days in April 2006, however, Chandra saw no X rays from the core. Researchers attribute the interruption to a cloud of material passing in front of the disk. From the duration of the eclipse and the inferred size of the cloud, the scientists determined that the proposed disk around this black hole must have a diameter seven times as great as the distance between Earth and the sun.

The Chandra observations also show that the eclipsing cloud lies only about a hundredth of a light-year from the black hole’s event horizon—the region that marks the boundary between the hole and the rest of the universe. Existing models of the regions around supermassive black holes don’t include much material at that distance, notes Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the Italian Institute for Astronomy in Florence. He and his colleagues report the findings in the April 20 Astrophysical Journal Letters.

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