Black hole seems caught in the same old groove   

 By Laura Bailey
 

Artist impression of hot gas orbiting a black hole. (Dana Berry/CfA/NASA)

ANN ARBOR, Mich.—A University of Michigan scientist is part of a team that discovered a black hole that has chiseled a stable dent in the fabric of space and time, like a dimple in one's favorite spot on a sofa.
The extent of this indentation may help scientists measure a black hole's mass and how it spins -- two long-sought measurements. Using NASA's Rossi X-ray Timing Explorer, the team of scientists saw identical patterns in the X-ray light emitted near the black hole over nine years, as captured in archived data from 1996 and in a new, unprecedented 550-hour observation from 2005.
“By understanding the behavior of matter so close to the black hole's edge, we can now begin to determine the spin and thus, for the first time, completely describe the black hole," said Jon Miller, a U-M assistant professor of astronomy.
"The fact that we found the exact same frequency of X-ray oscillations nine years later is likely no coincidence," added team leader Jeroen Homan of MIT's Kavli Institute. "The black hole is still singing the same tune. The oscillations are created by a groove hammered into spacetime by the black hole. This phenomenon has been suspected for a while, but now we have strong evidence to support it."
Homan and Miller, with Rudy Wijnands of Amsterdam University and Walter Lewin of MIT, presented their findings at the American Astronomical Society meeting in Washington in January.
Black hole regions are notoriously chaotic, generating light at a range of frequencies. Similarities seen nine years apart imply something fundamental is producing a pair of observed frequencies, namely the warping of space and time predicted by Einstein but rarely seen in such detail.
The team observed a region less than 100 miles from the event horizon of a black hole system called GRO J1655-40. Here, matter can orbit a black hole relatively stably, but occasionally it wobbles at certain precise frequencies. This is a direct result of how the black hole deforms space and time, a four-dimensional concept that Einstein called spacetime.

Artist impression of the black hole binary system GRO J1655-4. (Rob Hynes 2001)

For most of the time between the 1996-1997 outburst and the 2005 outburst, the source had fallen quiet as its supply of gas dried up. After the onset of the new outburst, the team observed GRO J1655-40 twice a day on average for eight months, for a total of over 550 hours. Gas from a companion star was falling toward the black hole, heating to high temperatures and causing the entire region to glow in X-ray light.
During the long observation the team uncovered fluctuations in the X-ray light, called quasi-periodic oscillations, or QPOs. These are thought to be from wobbling blobs of gas whipping around the black hole. The team observed QPOs at frequencies of 300 Hz and 450 Hz—the same that were observed nine years ago. This was by far the longest observation of a black hole during an outburst. Previous observations have determined that GRO J1655-40 is about 6.5 times more massive than our sun.
"The precise frequencies are determined by the mass of the black hole and also by how fast it spins," Miller said. "Those measurements—mass and spin—have been difficult to obtain.”

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