January 10, 2005
Matter Seen Speed-Racing Around a Black Hole
ESA -- Using a "Ëradar-gun' technique, typical of police speed-traps, scientists have clocked three separate clumps of hot iron gas whipping around a black hole at 30 000 kilometres per second, about a tenth of the speed of light.
The observation, made with ESA's XMM-Newton observatory, 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 (NASA Goddard Space Flight Center, Greenbelt, USA and University of Maryland Baltimore County, USA) presents this result today at a press conference at the American Astronomical Society in San Diego together with Dr Lance Miller (University of Oxford, United Kingdom).
"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."
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 in which 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 extremely high temperatures.
Turner's team observed a well-known galaxy named Markarian 766, located 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 disc. Flares erupt on this disc most likely when magnetic field lines emanating from the central black hole interact with regions on the disc.
To measure the speed of the flares and the black hole mass, scientists used a technique that involves measuring the Doppler shift and resembles that used by the police to catch speeding motorists. As an object moves towards us, the frequency or energy of its light rises. Conversely, the energy falls as the object moves away. This is the "ËDoppler effect' and a similar phenomenon happens with the changing pitch of a police siren. If it is approaching, the frequency of the sound is higher, but if it is receding the frequency is lower.
"We think we are viewing the accretion disc 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. By studying the pattern with which the light from the clumps rises and falls in energy, scientists could also determine the mass of the black hole and the viewing angle of the accretion disc.
With a known mass and orbital period, Turner and her team could determine the speed of the clumps using relatively simple Newtonian physics.
Two factors made the measurement possible. One is that XMM-Newton captured particularly persistent flares during a long observation, lasting nearly 27 hours. Equally crucial is the unprecedented light collecting power of XMM-Newton, which allowed scientists to look at how energy from the clumps changed over time.
Turner said this observation confirms a preliminary XMM-Newton result, announced in September 2004 by a European team led by Dr Kazushi Iwasawa of the Institute of Astronomy in Cambridge, United Kingdom, that something as detailed as an orbital period could be detected with the current generation of X-ray observatories. 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.
Videos and Animations
This animation depicts three hot chunks of matter orbiting a black hole.
If placed in our Solar System, this black hole would appear like a dark abyss spread out nearly as wide as Mercury's orbit. And the three chunks (each as large as the Sun) would be as far out as Jupiter. They orbit the black hole in a lightning-quick 30 000 kilometres per second, over a tenth of the speed of light.
This is a simplified illustration of two hot chunks of matter orbiting a black hole, showing how scientists tracked the blobs by observing their Doppler shift. First, we see one blob. Note how the energy emitted from this orbiting material rises to about 6.5 kilo-electron volt (an energy unit) as it moves towards us, and then falls to about 5.8 kilo-electron volt as it moves away.
This is the 'Doppler effect' and a similar phenomenon happens with the changing pitch of a police siren. If it is approaching, the frequency of the sound is higher, but if it is receding the frequency is lower. Matter goes round and round; energy goes up and down. About 14 seconds into the animation, a second blob is added, which also displays a rise and fall in energy during its orbit.
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