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January 10, 2005

Scientists See Matter Circling Black Hole at Breakneck Speed

UMBC Joint Center for Astrophysics scientists and their colleagues at Oxford University used a speed-gun technique typical of the highway patrol to clock three separate clumps of hot iron gas whipping around a black hole at 20,000 miles per second, over 10 percent of light speed.

The observation, made with the European Space Agency's XMM-Newton satellite, marks the first time scientists could trace individual blobs of shredded matter on a complete journey around a black hole. This provides a crucial measurement that has long been missing from black hole studies: an orbital period. Knowing this, scientists can measure black hole mass and other characteristics that have long eluded them.

Dr. Jane Turner, jointly affiliated with NASA Goddard Space Flight Center and the Joint Center for Astrophysics (JCA) at the University of Maryland Baltimore County (UMBC), presented this result today at a press conference at the American Astronomical Society in San Diego. Her co-presenter is Dr. Lance Miller of Oxford University.

"For years we have seen only the general commotion caused by massive black holes, that is, a terrific outpouring of light," said Turner. "We could not track the specifics. Now, with XMM-Newton, we can filter through all that light and find patterns that reveal information about black holes never seen before in such clarity."

Dr. Miller noted that if this black hole were placed in our Solar System, it would appear like a dark abyss spread out nearly as wide as Mercury's orbit. And the three clumps of matter detected would be as far out as Jupiter. They orbit the black hole in a lightning-quick 27 hours (compared to the 12 years it takes Jupiter to orbit the Sun).

Black holes are regions in space so dense that gravity prevents all matter and light from escaping. What scientists see is not the black hole itself but rather the light emitted close to it as matter falls towards the black hole and heats to high temperatures.

Turner's team observed a well-known galaxy named Markarian 766, about 170 million light years away in the constellation Coma Berenices (Bernice's Hair). The black hole in Markarian 766 is relatively small although highly active. Its mass is a few million times that of the Sun; other central black hole systems are over 100 million solar masses.

Matter funnels into this black hole like water swirling down a drain, forming what scientists call an accretion disk. Flares erupt on this disk most likely when magnetic field lines emanating from the central black hole interact with regions on the disk.

"Calculating the flares' speeds and the black hole mass was straightforward, based on Doppler shifting, the technique used by law officers to nab speeders." said Dr. Ian George of UMBC's JCA and NASA Goddard, a co-author on a scientific journal article the team has prepared. "Light appears to rise in energy as an object moves towards us and then fall in energy as it moves away. A similar phenomenon happens with the sound of a passing car on a highway, going 'eeeeeeyyoool.'"

"We think we're viewing the accretion disk at a slightly tilted angle, so we see the light from each of these flares rise and fall in energy as they orbit the black hole," Miller said.

When the scientists made a graph of energy (on the y-axis) and time (on the x-axis), they saw near-perfect sinusoidal curves from each of the three clumps of matter they observed. The width, or period, of the curves is proportional to black hole mass. The height of the curves is related to the viewing angle of the accretion disk. With a known mass and orbital period, the scientists could determine velocity using relatively simple Newtonian physics.

Two factors made the measurement possible. The scientists observed particularly persistent flares during a long observation, nearly 27 hours. Also, "no telescope before XMM-Newton has had the light-collecting power to allow for a comparison of energy over time," said Dr. James Reeves of NASA Goddard, also part of the team.

Turner said this observation confirms a preliminary XMM-Newton result announced by a European team in September -- that something as detailed as an orbital period could be detected with the current generation of X-ray telescopes. The combination of results indicates that scientists, given long observation times, are now able to make careful black hole measurements and even test general relativity in the domain of extreme gravity.

Posted by crose at January 10, 2005 4:54 PM