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How Hungry is a Black Hole?

X-rays from distant galaxies may reveal the appetites of giant black holes that lurk there. As x-ray astronomers report in the current issue of Astrophysical Journal Letters, the intensity of one particular x-ray wavelength may show how fast a black hole is devouring matter from its surroundings: the weaker the x-rays, the more voracious the hole.

These black holes, weighing up to a billion times more than our sun, are thought to be the power sources for brilliant "active galaxies," which include objects called quasars that can radiate more energy than anything else in the universe. The idea is that matter sucked into the black hole's crushing gravitational field gives off light and other radiation as it is heated to millions of degrees. But because the black holes are hidden at the centers of galaxies tens of millions to billions of light-years away and give off no light of their own, they have not been easy to study directly.

Paul Nandra of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says that x-rays from the superhot material can reveal "conditions right next to supermassive black holes in the most distant galaxies known." He and his colleagues focused on the K-alpha emission line of iron, visible in the spectra of many active galactic nuclei (AGN)--the brilliant centers of these galaxies. When high-energy radiation generated by matter falling into a black hole dislodges an inner electron from an iron atom, an outer electron drops to the unoccupied orbital and emits an x-ray photon at the K-alpha wavelength.

Nandra and his colleagues compared the K-alpha line from 39 AGN's, including those from 21 quasars, and found a clear trend: More luminous AGN's have smaller K-alpha emission lines, and the line vanishes in the brightest ones. The astronomers argue that the intensity falls when more of the iron in matter near the black hole is completely ionized--stripped of all 26 of its electrons, so none remain to generate the emission line. The ionization increases, the astronomers say, near black holes that are swallowing up matter at a higher rate relative to their mass, which creates more radiation capable of stripping away electrons.

While some investigators have thought that the overall mass of a black hole is what determines its brilliance, Nandra's finding supports a competing idea that appetite is the key factor. Says astrophysicist Mitch Begelman of the University of Colorado, Boulder, "finally we're able to put this on a quantitative footing, rather than just simply saying that this sort of trend wouldn't be surprising."