Magazine article Science News

Finding the Origins of the X-Ray Sky

Magazine article Science News

Finding the Origins of the X-Ray Sky

Article excerpt

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. …

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