Wednesday, November 6, 2013

Black holes found in globular star clusters, upsetting 40 years of theory

November 6, 2013
A  black hole was discovered in the M62 star cluster, which is 23,000 light years away from Earth (credit: Texas Tech University)
Astrophysicists have discovered another example of a black hole in a globular star cluster for a total of three, upsetting 40 years of theories against their possible existence.
Tom Maccarone, a Texas Tech University associate professor of physics, said globular star clusters are large groupings of stars thought to contain some of the oldest stars in the universe and could have a million to tens of millions of stars, Maccarone said.
“The stars can collide with one another in that environment,” Maccarone said. “The old theory believed that the interaction of stars was thought to kick out any black holes that formed.” Maccarone said black holes might still get kicked out of globular star clusters, but at a much slower rate than initially believed.
New black Hole Spotted
VLA radio image (left) and Chandra x-ray image (right). The yellow circle in each image shows the black hole found in the M62 star cluster in our Galaxy. The red circle denotes a neutron star close by. (Credit: Texas Tech University)
“As the black hole eats a star, these jets of material are coming out,” he said. “Most of the material falls into the black hole, but some is thrown outwards in a jet. To see that jet of material, we look for a radio emission. We found a few radio emissions coming from this globular star cluster that we couldn’t explain any other way.”
Maccarone said black holes in globular clusters may get close enough to one another to merge into bigger black holes. “These mergers may produce the ‘ripples in spacetime’ we call gravitational waves,” he said. “Trying to detect gravitational waves is one of the biggest problems in physics right now, because it would be the strongest test of whether Einstein’s theory of relativity is correct.”
Other researchers included Laura Chomiuk and Jay Strader at Michigan State University; James Miller-Jones at Perth Curtin University in Perth, Australia; Craig Heinke at University of Alberta in Edmonton, Alberta, Canada; Eva Noyola at the University of Texas at Austin; Anil Seth at University of Utah; and Scott Ransom at the National Radio Astronomy Observatory in Charlottesville, Virginia.
Abstract of The Astrophysical Journal paper
We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we call M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source with a flux density of 18.7 ± 1.9 μJy at 6.2 GHz and a flat radio spectrum (α = –0.24 ± 0.42, for S ν = να). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated with a Chandra X-ray source, supporting its identification as a black hole X-ray binary. Measurements of its radio and X-ray luminosity, while not simultaneous, place M62-VLA1 squarely on the well-established radio-X-ray correlation for stellar-mass black holes. In archival Hubble Space Telescope imaging, M62-VLA1 is coincident with a star near the lower red giant branch. This possible optical counterpart shows a blue excess, Hα emission, and optical variability. The radio, X-ray, and optical properties of M62-VLA1 are very similar to those for V404 Cyg, one of the best-studied quiescent stellar-mass black holes. We cannot yet rule out alternative scenarios for the radio source, such as a flaring neutron star or background galaxy; future observations are necessary to determine whether M62-VLA1 is indeed an accreting stellar-mass black hole.

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