Finding the origins of the X-ray sky.

Point an X-ray telescope at a seemingly blank patch of sky and the detector will find a region ablaze with radiation. Astronomers discovered this cosmic sea of radiation, known as the X-ray background, in 1962. Researchers have debated its origins ever since.

Scientists have generally agreed that radiation pouring out of active galactic nuclei (AGNs) - quasars, Seyfert galaxies, and other compact sources - accounts for at least half the low-energy portion of the X-ray background. As in a Seurat painting, these point-like sources of light combine to form the broad brush strokes of the low-energy X-ray sky. But the bulk of the X-ray background - the high-energy spectrum above a few thousand electron-volts (eV) - remained a mystery. The few observations of AGN's didn't seem to account for the high-energy part of the background. Other sources, such as a proposed sea of hot intergalactic gas, proved false.

Based on newly analyzed data from three spacecraft, astronomers now report that emissions from AGN's do indeed match the high-energy X-ray background. Julian H. Krolik of Johns Hopkins University in Baltimore and his Polish colleagues. Andrej A. Zdziarski and Piotr T. Zycki of the Copernicus Astronomical Center in Warsaw, detail their work in the Sept. 10 Astro-physical Journal Letters.

Krolik says researchers doubted that AGNs created the bulk of the X-ray background, because they thought each of these powerhouse emitted too little radiation at energies of 10,000 to 20,000 eV and too much at higher energies.

But new data tell a different story about the behavior of AGNs. Reviewing studies conducted with the Japanese satellite Ginga, Krolik's team noticed that AGNs emit more X-rays at about 10,000 eV than previously thought. More significantly, among the AGNs detected at high X-ray energies by NASA's Compton Gamma Ray Observatory, most declined dramatically in brightness between 50,000 and 100,000 eV, Krolik says. Observations with the Russian satellite GRANAT support this finding, he adds.

At first glance, the decline in AGN emission still wouldn't seem to account for the X-ray background, which takes a nosedive at a lower energy. But Krolik notes that the NASA craft could only detect nearby sources. Because more distant bodies recede faster than neighboring ones in an expanding universe, faraway AGNs would appear to have their output shifted to lower energies. Thus, distant AGNs would seem to drop in intensity at a lower energy than those closer. In this way, emissions from the distant powerhouses "provide an excellent fit to both the intensity and spectral shape of the X-ray background," the team writes.

Krolik emphasizes that the study assumes that faraway AGNs behave similarly to the nearer population.

"This is the best [analysis] I've ever seen," says X-ray astronomer Stephen S. Holt of NASA's Goddard Space Flight Center in Greenbelt, Md. "I'm almost willing to say now that AGN's create the background."