Researchers Report X-ray Detection of Binary Black Holes in Galaxy Cluster Abell 400

5/22/2006 - 24-06r
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An international team of astrophysicists from the University of Bonn in Germany, the Naval Research Laboratory and the University of Virginia has detected a proto supermassive binary black hole in images from NASA's Chandra X-ray Observatory. The scientists found that these two black holes are gravitationally bound and orbit each other. Their results will be published in an upcoming issue of Astronomy and Astrophysics.

"The two black holes have been previously seen in radio images," notes NRL team member Tracy Clarke of the Laboratory's Remote Sensing Division, "however these findings report the first X-ray detection of these black holes."

The two black holes are located in the nearby galaxy cluster Abell 400. Using high-resolution Chandra data, the team was able to spatially resolve the two supermassive black holes (separated by 15") at the center of the cluster. Each black hole is located at the center of its respective host galaxy and the host galaxies appear to be merging. However, the two host galaxies are not only colliding - the whole cluster in which they live is merging into a neighboring galaxy cluster.

Using these new data, the team shows that the two black holes are moving through the intracluster medium at the supersonic speed of about 1200 kilometers per second. The scientists believe that the wind from this motion causes the radio plasma emitted from these two black holes to bend backwards. Although this bending had been observed previously, the cause was still under debate. Since the bending of the jets due to this motion is in the same direction, it suggests that the two black holes are travelling along the same path within the cluster and are therefore gravitationally bound.

These two black holes became gravitationally bound when their host galaxies collided. In several million years, the two black holes will likely combine, causing a burst of gravitational waves. This event would produce one of the brightest sources of gravitational radiation in the Universe.

The observations provide additional evidence that such bound systems exist and are currently merging. These mergers are believed to be the biggest source of gravitational waves that NASA's Laser Interferometer Space Antenna, scheduled to launch in 2015, will detect.

The research team was led by Dr. Daniel Hudson of the University of Bonn. Collaborators included Dr. Thomas Reiprich, also of the University of Bonn; Dr. Craig Sarazin, University of Virginia; and Tracy Clarke of NRL.

These images show the central region of the galaxy cluster Abell 400. The color coding gives the temperature of the X-ray emitting gas trapped in the cluster: black-cold (18 million degrees Celsius) to white-hot (38 million degrees Celsius). The contours show the radio emission from the jets of plasma being expelled by the black holes. As the two black holes stream through the gas at supersonic velocities, the jets are bent toward the top of the image. The gas in front of the black holes is compressed and heated, as seen by the hotspot below them. The inset shows a blow-up of the central regions. Each dot represents a position where an X-ray photon has struck Chandra's X-ray camera. The two black holes are seen as bright regions where as many as 250 X-ray photons struck the camera. The contours again show the radio emission from the black holes and the jets of plasma being ejected from them.

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