Astronomers see huge clouds of gas orbiting supermassive black holes at the centres of galaxies. Once thought to be a relatively uniform, fog-like ring, the accreting matter instead forms clumps dense enough to intermittently dim the intense radiation blazing forth as these enormous objects condense and consume matter.
The international team reports their sightings in a paper to be published in the Monthly Notices of the Royal Astronomical Society.
Evidence for the clouds comes from records collected over 16 years by NASA's Rossi X-ray Timing Explorer, a satellite in low-earth orbit equipped with instruments that measured variations in X-ray sources. Those sources include active galactic nuclei, brilliantly luminous objects powered by supermassive black holes as they gather and condense huge quantities of dust and gas.
By sifting through records for 55 active galactic nuclei Alex Markowitz, an astrophysicist at the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany and colleagues found a dozen instances when the X-ray signal dimmed for periods of time ranging from hours to years, presumably when a cloud of dense gas passed between the source and satellite.
Mirko Krumpe of the European Southern Observatory in Garching, Germany and Robert Nikutta, of Andrιs Bello University in Santiago, Chile co-authored the report, which confirms what recent models of these systems have predicted.
The clouds they observed orbit a few light-weeks to a few light-years from the centre of the active galactic nuclei. One, in a spiral galaxy in the direction of the constellation Centaurus designated NGC 3783, appeared to be in the midst of being torn apart by tidal forces.
Support for this research came from NASA's Astrophysics Data Analysis Program (NNX11AD07G) and the European Community's Seventh Framework Program (229517). Nikutta acknowledges support from ALMA-CONICYT (31110001). Video produced by the Scientific Visualization Studio, Goddard Spaceflight Centre, NASA, based in part on visualisations created by Wolfgang Steffen, Institute of Astronomy, National Autonomous University of Mexico.
- A. G. Markowitz, M. Krumpe, R. Nikutta. First X-ray-based statistical tests for clumpy-torus models: eclipse events from 230 years of monitoring of Seyfert AGN. Monthly Notices of the Royal Astronomical Society, 2014; DOI: 10.1093/mnras/stt2492
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