Spinning black hole leaves dent in space-time (Forwarded)

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January 9, 2006

Spinning black hole leaves dent in space-time

MIT scientists and colleagues have found a black hole that has chiseled
a remarkably stable indentation in the fabric of space and time, like a
dimple in one's favorite spot on the sofa.

The finding may help scientists 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 saw
identical patterns in the X-ray light emitted near the black hole nine
years apart, 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 very
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 the MIT Kavli Institute and his colleagues from the
University of Michigan, Amsterdam University and MIT are presenting this
result today at the annual meeting of the American Astronomical Society
in Washington, D.C.

"The fact that we found the exact same frequency of X-ray oscillations
nine years later is likely no coincidence," said Homan. "The black hole
is still singing the same tune. The oscillations are created by a groove
hammered into space-time by the black hole. This phenomenon has been
suspected for a while, but now we have strong 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 -- which includes Jon Miller of the University of Michigan,
Rudy Wijnands of Amsterdam University and Walter Lewin of MIT --
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 space-time.

The team observed GRO J1655-40 twice a day on average for eight months,
for a total of more than 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
as those 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
the sun.

"The precise frequencies are determined by the mass of the black hole
and also by how fast it spins," said Miller. "Those measurements -- mass
and spin -- have been difficult to obtain. Fortunately, we already have
an estimate of the mass of this black hole. 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."

Making this detection possible, the team said, was the long and
intensive observing program with the Rossi X-ray Timing Explorer, a
unique and durable observatory launched on Dec. 30, 1995.

"Had we not observed in this way, we would probably not have detected
the pair of QPOs again," said Wijnands. "We need time. X-ray light from
black holes typically shows many types of fluctuations. Often we see
black holes brighten and weaken a few times per second, but the rate at
which this happens changes from day to day. What is so special about the
fluctuations that we observed is not only that they are much faster than
the ordinary fluctuations -- a few hundred times per second! -- but also
that the rate of the fluctuations is exactly the same as when we last
saw them, nine years ago."

For more information about this result, including images, visit
http://tahti.mit.edu/blackhole

RELATED

* Scientists find black hole's 'point of no return'
http://web.mit.edu/newsoffice/2006/blackhole1.html
* Spacetime wave orbits black hole
http://web.mit.edu/newsoffice/2005/spacetime.html
* MIT Kavli Institute for Astrophysics and Space Research
http://space.mit.edu/

IMAGE CAPTION:
[http://web.mit.edu/newsoffice/2006/b...2-enlarged.jpg (44KB)]
This graphic depicts hot gas orbiting a black hole.

Image: Dana Berry/CfA/NASA