The catalog is unusual because it has been compiled using XMM-Newton's "spare time" - keeping the spacecraft's cameras open as it slews between different sky targets. The result, ESA astronomers said Wednesday, is a "free-of-charge" survey that has produced data in unprecedented detail.

So far, astronomers have identified about 55 percent of the X-ray sources in the catalog as known stars, galaxies, quasars and clusters of galaxies.

XMM-Newton's ability to slew rapidly means a star or galaxy passes in front of the telescope's field of view for only about 10 seconds. Nevertheless, the spacecraft's collecting area, coupled with the efficiency of its image sensors, has detected thousands of new X-ray sources.

Furthermore, by comparing XMM-Newton survey's data with those obtained over a decade ago by the international all-sky ROSAT mission, astronomers now can check the long-term stability of about 2,000 objects.

An initial look shows some sources have changed their brightness level in X-ray wavelengths by significant amounts. The most extreme are variable stars and, more surprising, galaxies with unusual volatility that may be due to large quantities of matter being consumed by an otherwise dormant central black hole.

The slew survey is particularly sensitive to active galactic nuclei - galaxies with unusually bright centers - that have been observed to distances of about 10 billion light-years.

Most stars and galaxies look like points in the sky, but about 15 percent of the sources cataloged by XMM-Newton exhibit an extended X-ray emission. Most are clusters of galaxies - gigantic conglomerations of stars that trap hot gas and emit X-rays over scales of 1 million light-years.

Of these clusters, 81 already are known from earlier research, but XMM-Newton has detected many previously unknown clusters.

Astronomers hope the newly detected sources of this kind also include very distant clusters that are highly luminous in X-rays, a characteristic that makes them invaluable for investigating the evolution of the universe.

Now, large optical telescopes need to conduct follow-up observations to determine the distances of the individual galaxies in the newly discovered clusters.

Using traditional pointed observations, it takes huge amounts of telescope time to image very large sky features, such as old supernova remnants in their entirety. The slewing mechanism provides a very efficient method of mapping these objects, and several have been imaged including the 20 000 year-old Vela supernova remnant, which occupies a sky area 150 times larger than the full Moon.

Extraordinarily bright, low-mass X-ray binary systems of stars, called LMXBs - either powered by matter pulled from a normal star or exploding onto the surface of a neutron star, or being consumed by a black hole - can be observed with sufficient sensitivity to record their detailed light spectrum. Passes across these intense X-ray sources can help astronomers understand the long-term physics of the interaction between two stars in a binary system.

The next step is to turn around the slewing data more quickly, so astronomers might be able to catch interesting transient X-ray targets before they have a chance to fade. In addition, the research team plans to continue to update the catalog, eventually covering at least 80 percent of the sky.

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