Black holes in a double pack

November 21, 2002

The extraordinarily bright galaxy NGC 6240 is roughly 400 million light years away from earth and is a prime example of the collision and subsequent merging of two galaxies causing "fireworks" as new stars are created. The center of this galaxy is hidden behind innumerable dusty gas clouds and is therefore not visible with optical telescopes. X-rays can, however, penetrate the veil of gas and dust.

Previous observations have shown that NGC 6240 produces high energy X-radiation. Using radio, infrared, and optical observations, astronomers detected two bright nuclei in this system, whose nature remained a mystery. "With Chandra, we hoped to determine which of the two nuclei, if any, contains an active super massive black hole," says Stefanie Komossa from the Max Planck Institute for Extraterrestrial Physics and lead author of the paper on NGC 6240 that is soon to be published in the Astrophysical Journal Letters. NASA's Chandra observed NGC 6240 with the Advanced CCD Imaging Spectrometer (ACIS) for a total of 10.3 hours.

"Much to our surprise, we found that both nuclei harbor active black holes", explains Komossa. The detection of a binary black hole supports the idea that black holes can grow to become enormously massive in the centers of galaxies by merging with other black holes. "This is important for our understanding of how galaxies form and evolve."

"The breakthrough came with Chandra's ability to clearly distinguish the two nuclei and measure the details of the X-radiation from each nucleus," says Guenther Hasinger, director at the Max Planck Institute for Extraterrestrial Physics and co-author of the paper. According to Hasinger, both active cosmic monsters leave fingerprints. "We have observed an excess of high energy photons from hot gas swirling around a black hole and X-rays from fluorescing iron atoms in gas near the black hole."

Over the next few hundred million years, the two black holes in NGC 6240, which are roughly 3000 light years apart, will drift toward each other and eventually merge to form an even larger, supermassive black hole. The process will end several hundred million years from now with an enormous burst of gravitational waves. These gravitational waves will spread through the universe and produce ripples in the fabric of space, which will appear as minute changes in the distance between any two points.

The merging of two super massive black holes like those in NGC 6240 will create the most powerful gravitational waves in the universe. LISA (Laser Interferometer Space Antenna), the space-based detector planned by NASA and ESA, will search for gravitational waves from massive black hole mergers. Such events are estimated to occur several times each year in the observable universe. "This is the first time that we see a binary black hole in action, the smoking gun evidence for something which will become a major gravitational wave burst in the future", says Hasinger.
Other members of the team include Vadim Burwitz and Peter Predehl from the Max Planck Institute for Extraterrestrial Physics, Jelle Kaastra from the Space Research Organization in the Netherlands, and Yasushi Ikebe from the University of Maryland in Baltimore.

NASA's Marshall Space Flight Center in Huntsville, Alabama manages the Chandra program for the Office of Space Science in Washington. The company TRW in Redondo Beach, California is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts. The Deutsches Zentrum fuer Luft- und Raumfahrt (DLR) funded Germany's contribution to Chandra.

Images and additional information are available online at:,,


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