Variable quasar may help measure the cosmos.

A flickering cosmic mirage, recorded for the first time in X rays, promises to provide a new estimate of how rapidly the universe is expanding.

The mirage is generated by gravity. A single source of light, such as the beacon of a distant quasar, appears to an observer as several images when a galaxy or other massive body lies directly between the source and Earth. The gravity of the intervening body acts like a lens, bending the light rays from the quasar into separate images. The light beams from each image travel along slightly different paths, thereby taking different amounts of time to reach Earth.

If the quasar suddenly brightens, the flicker appears in one image before it shows up in another. Like cosmic surveyors, astronomers determine the distance to the intervening galaxy by using the time delays between brightenings and a model for the distribution of mass within the lens. From that calculation, they can pin down a value for the Hubble constant, a measure of how rapidly the universe is expanding. This method is more straightforward than others, which rely on assumptions such as estimates of star brightness and distances to nearby galaxies (SN: 5/29/99, p. 340).

This promising approach hasn't yet been fruitful because in visible light, quasars typically take months to years to flicker. So, it can take years to make visible-light measurements of time delays.

Better would be measurements of quasars at X-ray wavelengths where the beacons fluctuate rapidly. That's just what astronomers have finally begun doing.

Using NASA's Chandra X-ray Observatory, George Chartas of Pennsylvania State University in University Park and his colleagues recently found that light from the quasar RXJ0911.4+0551 is split into four X-ray images. Moreover, they found that the quasar flickered in one image on a time scale of less than 1 hour. He and his collaborators described the findings on Nov. 7 in Honolulu at a meeting of the American Astronomical Society.

During the initial survey, the team's observation period was too short to record a flicker in more than one image. But a single observation lasting several hours would give astronomers the chance to measure a delay, says Chartas.

The researchers have obtained X-ray images of several other quasars that undergo lensing. If all of them flicker as rapidly as RXJ0911.4+0551 does, astronomers will be able to derive a new measurement of the Hubble constant, Chartas says.

None of this will be easy, notes Christopher S. Kochanek of Harvard University. While he welcomes time-delay measurements that are more accurate, he says astronomers must better determine how mass is distributed in each lens before they can convert the data into a new estimate of the Hubble constant.