Makeup of the X-ray background
Credit: NASA/Goddard Space Flight Center

Parsing the Background

The entire sky glows in X-rays. No one really knows why. Unlike its (perhaps) more famous younger cousin, the cosmic microwave background, scientists believe that the X-ray background is not really a diffuse glow beyond the Milky Way, but is instead mostly composed of X-ray emission from very faint individual supermassive central black holes in far away active galaxies. And if we had super-large hard X-ray telescopes capable of resolving these sources spatially and detecting them no matter how faint, we'd be able to see these individual galaxies for study, instead of the combined haze of X-rays we see now. One way to help resolve this issue is to try to constrain the numbers of active galaxies with heavily obscured central back holes. But these supermassive black holes can only be detected at very high X-ray energies, because we view them edge on through a thick obscuring torus of gas and dust. But using the high-energy telescope on the Swift Gamma-ray burst hunter, scientists have been able to constrain the numbers of such hidden supermassive black holes in our local universe. And surprisingly these sources seem to contribute the largest amount to the X-ray background, as shown in the schematic above.
Published: January 31, 2011

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Page Author: Dr. Michael F. Corcoran
Last modified Sunday, 06-Feb-2011 18:48:09 EST