Energy from a Black Hole
Posted: Tuesday, October 23, 2001
There are plenty of black holes that gobble energy. Now astronomers have spotted one in a distant galaxy that's giving some of its energy back.
Scientists for the first time have seen energy being extracted from a black hole. Like an electric dynamo, this black hole spins and pumps energy out through cable-like magnetic field lines into the chaotic gas whipping around it, making the gas -- already infernally hot from the sheer force of crushing gravity -- even hotter.
Joern Wilms of Tuebingen University, Germany, and an international team of astronomers observed the novel "power tapping" with the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton) satellite by watching a supermassive black hole in the core of a galaxy named MCG-6-30-15. The observation also may explain the origin of particle jets in quasars.
"Never before have we seen energy extracted from a black hole," said co-author Christopher Reynolds of the University of Maryland, College Park. "We always see energy going in, not out."
"The gravity in this region appears to be so intense that the very fabric of space twists around the black hole, dragging magnetic field lines along with it," said Wilms. "The magnetic fields tighten about the black hole, slowing its spin. This 'friction' heats the region to even higher temperatures."
Scientists say most galaxies, including our Milky Way galaxy, have a supermassive black hole at their core. A supermassive black hole contains the mass of millions to billions of Suns compressed within a region smaller than our solar system. The black hole in MCG-6-30-15, over 100 million light-years from Earth, has the mass of about 100 million Suns.
The team observed the X-ray glow of iron gas traveling about half the speed of light very close to the event horizon of the black hole in MCG-6-30-15 (an "event horizon" is the theoretical border of a black hole). XMM-Newton captured the spectrum, or chemical fingerprint, of this gas. The spectrum, laid out on a graph, resembles an electrocardiogram with its spikes and dips.
The iron spectrum from MCG-6-30-15 has extremely broad "spikes," an indication of gravity tugging at the particles of light, called photons, and literally stretching the light. MCG 6-30-15's iron line was so broad, in fact, that the bulk of the light must emanate from very close to the black hole, where the force of gravity is the greatest, Reynolds said.
The total energy output, or luminosity indicated by the spectrum, however, was too bright to be powered by gravity and the free fall of matter alone. Some additional power source must exist to boost the luminosity to the observed intensity.
Co-author Mitchell Begelman of the University of Colorado said this finding may be observational evidence of a theory by Professor Roger Blandford, currently at the California Institute of Technology, and Dr. Roman Znajek, when he was at Cambridge University in England, over 25 years ago. According to the theory, rotational energy can be extracted from the black hole as it is braked by magnetic fields.
Begelman said the energy lost in MCG-6-30-15 is transferred to the inner edge of the accretion disk, a flow of gas swirling around and eventually falling into the black hole. The Blandford-Znajek theory implies that energy flows to particle jets emanating perpendicularly from the accretion disk in certain supermassive black hole systems called quasars. MCG 6-30-15 is not a quasar, but Begelman said the theory can still apply because it predicts that the magnetic field might also link to the disk.
ASCA, a Japanese X-ray satellite, found possible evidence of a spinning black hole in 1994, but the signal was too weak to observe any evidence of energy being extracted from the black hole.
XMM-Newton, launched from French Guiana by ESA in December 1999, carries three advanced X-ray telescopes with the light-collecting ability to detect millions of sources, far greater than any previous X-ray mission.
The advanced X-ray observing abilities of this satellite made this important discovery about the energy dynamics of black holes possible, and should lead to other exciting discoveries about our cosmos in the future.
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