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.