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X-ray study: Energy from a black hole?

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10:12am, October 31, 2001
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Black holes are the ultimate suck-ups. Everything–even light–that comes near the crushing gravity of these ultradense bodies falls in, and nothing gets out.

Scientists now claim that for the first time, they've observed energy extracted from a black hole, or more precisely, from the whirl of surrounding space that, according to previous studies, a spinning black hole drags along with it (SN: 11/15/97, p. 308).

Jörn Wilms of the University of Tübingen in Germany and his colleagues, who include Christopher S. Reynolds of the University of Maryland in College Park, base their findings on studies of a supermassive black hole about 100 million times the sun's mass. It lies 100 million light-years from Earth and within the core of the galaxy MCG-6–30–15.

Using the European Space Agency's X-ray Multi-Mirror Mission-Newton (XMM-Newton) satellite, the team observed emissions from ionized iron atoms just outside the hole. Several features of the emissions suggest the spinning black hole has given up some of its rotational energy to the ions, Wilms and his colleagues report in an upcoming Monthly Notices of the Royal Astronomical Society.

For years, astronomers have observed X rays from the regions around suspected black holes. The radiation comes from the swirling disk of gas that forms there. As gas particles fall toward the hole, they lose gravitational energy, ultimately converting some of it into heat and X rays. This process occurs throughout the disk, but a spectrum taken by XMM-Newton indicates that the X-ray emissions from the iron ions primarily come from just one part.

One of the spikes in the iron-ion spectrum provides a strong clue for this localization. The spike is extremely broad, indicating that the ions whip around the hole very rapidly and experience an extraordinarily strong gravitational tug.

The observed broadening is only possible, the researchers say, at the inner rim, the swiftest part of the disk.

Moreover, the X rays are too bright to be explained solely by the gravitational infall of gas. Some extra energy source must heat the ions, the astronomers surmise.

One explanation is that the spinning black hole and its surroundings transfer rotational energy to these iron ions. Theorists have long predicted that such energy extraction could be achieved via magnetic fields linking the disk to the hole.

In their 1977 proposal, Roger D. Blandford, now at the California Institute of Technology in Pasadena, and Roman Znajek of the University of Cambridge in England noted that the churning of its ionized gas gives the disk a large magnetic field. One end of each of the ropelike magnetic field lines remains anchored in the disk, while the other is carried off by gas particles as they exit the disk and fall into the hole.

Because the end of each magnetic field line tied to the hole whips around faster than the end anchored in the disk, there's tension that slows the black hole's spin and pumps the energy into the inner part of the disk. The energy drives the ions there to glow brighter.

Despite the match between theory and data, Blandford cautions the idea remains controversial. Coleman Miller of the University of Maryland is more enthusiastic. He says the findings provide both evidence that supermassive black holes spin and a new way to elucidate their workings.

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