NEW ORLEANS, LOUISIANA--Space artists love to show matter spiraling into the bottomless maw of a black hole, like water down a drain. Astronomers now think they've caught a glimpse of that far-out process for real. An x-ray telescope has spotted repeated gasps of light from hot gas at the center of a galaxy, an apparent signature of several orbits around a giant black hole in the span of one day. However, the complex analysis hasn't convinced everyone.
The standard picture of active black holes shows them enveloped by a torus of matter, called an accretion disk. Close to the hole, gas in the disk spins so fast that it heats up to millions of degrees and throws off x-rays. These x-rays get stretched to longer wavelengths as they fight to escape the hole's strong gravity. The frequency of the radiation also shifts back and forth as the gas spins rapidly toward and away from us, the familiar Doppler effect. In 2001, Europe's XMM-Newton satellite recorded both of these processes in x-rays from the core of a galaxy called NGC 3516, strong evidence for a whirling blaze of doomed matter.
In the new study, a team reanalyzed the XMM signals to search for hidden patterns. Astrophysicist Kazushi Iwasawa of the University of Cambridge, U.K., and colleagues found signs of a process never before seen in an accretion disk: a single hot spot whipping around the black hole nearly four times during XMM's daylong exposure. A cyclical sawtooth pattern in the x-ray energies revealed the orbits, Iwasawa says. The team's analysis suggests that the hot parcel of gas--probably iron atoms heated by a nearby flare of magnetic energy--orbited at 1/5th the speed of light, at about the same distance from the black hole as Earth's distance from the sun. Iwasawa reported the results here on 9 September at a meeting of the American Astronomical Society's High-Energy Astrophysics Division.
The work is "extremely exciting," says astrophysicist Paul Nandra of Imperial College in London. "This is one of our best opportunities to probe a strong gravitational field." But astronomer Jane Turner of the University of Maryland in Baltimore County, who led the earlier study of energy variations from NGC 3516, suspects the regular pattern isn't real. "It boils down to whether [the variations] are random or periodic," Turner says. "The periodicity is not statistically compelling, because there aren't enough cycles."