The Cloudy Cores of Active Galaxies

At the hearts of most big galaxies, including our own Milky Way, there lurks a supermassive black hole weighing millions to billions of times the sun's mass. As gas falls toward a supermassive black hole, it gathers into a so-called accretion disk and becomes compressed and heated, ultimately emitting X-rays. The centers of some galaxies produce unusually powerful emission that exceeds the sun's energy output by billions of times. These are active galactic nuclei, or AGN.

Using data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite, an international team has uncovered a dozen instances where X-ray signals from active galaxies dimmed as a result of a cloud of gas moving across our line of sight. The new study triples the number of cloud events previously identified in the 16-year archive.

The study is the first statistical survey of the environments around supermassive black holes and is the longest-running AGN-monitoring study yet performed in X-rays. Scientists determined various properties of the occulting clouds, which vary in size and shape but average 4 billion miles (6.5 billion km) across – greater than Pluto's distance from the sun — and twice the mass of Earth. They orbit a few light-weeks to a few light-years from the black hole.

See http://www.nasa.gov/content/goddard/rxte-reveals-the-cloudy-cores-of-active-galaxies/