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 finding may help researchers measure a black hole's mass and how it spins, two long-sought measurements, by virtue of the extent of this indentation. Using NASA's Rossi X-ray Timing Explorer, the team of researchers 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.

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.

Jeroen Homan of MIT's Kavli Institute and his team, which included Jon Miller of U-M, Rudy Wijnands of Amsterdam University and Walter Lewin of MIT, presented the result at the American Astronomical Society meeting in Washington this week.

"The fact that we found the exact same frequency of X-ray oscillations nine years later is likely no coincidence," Homan said. "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 good evidence to support it."

A black hole forms when a very massive star runs out of fuel. Without the power to support its mass, the star implodes and the core collapses to a point of infinite density. Black holes have a theoretical border called an event horizon. Gravity is so strong within the event horizon that nothing, not even light, can escape its pull. Outside the event horizon, light can still escape.

Homan's 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.

During most of the time between the 1996-1997 outburst and the 2005 outburst, the source had fallen into quiescence. The 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 believed 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. Prior observations have determined that GRO J1655-40 is about 6.5 times more massive than the sun.



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