Gamma-ray bursts surpass quasars as most distant probes

Gamma-ray bursts and their X-ray and optical afterglows have surpassed quasars as the most distant probes of the early universe, according to new calculations performed by Chicago astronomers.

Don Lamb Jr., Professor in Astronomy & Astrophysics, described the cosmological implications of gamma-ray bursts on Friday, Oct. 22, at the Fifth Huntsville International Symposium on Gamma-ray Bursts in Huntsville, Ala. His work and that of co-author Daniel Reichart, a Chicago graduate student, builds on findings published last month by Shrinivas Kulkarni and Joshua Bloom of the California Institute of Technology regarding the potential origins of gamma-ray bursts.

“Gamma-ray bursts may be beacons flashing us messages about the early universe,” Lamb said. It takes light from quasars billions of years to reach Earth, but gamma-ray bursts apparently go back even farther. If such bursts really exist beyond the range of quasars, then NASA’s High Energy Transient Explorer-2 satellite, scheduled for launch Jan. 23, and the newly announced Swift mission in 2003 should be able to detect them, he said.

Detecting the most distant gamma-ray bursts could provide a bonanza of cosmological data, including the first glimpse of star formation in the universe, Lamb said.

The origin of these bursts has remained a mystery since their discovery more than 30 years ago. The bursts occur almost daily and shine at least a billion times brighter than any other phenomenon in the universe, including quasars. The bursts last anywhere from a few milliseconds to several minutes, then disappear forever. The bursts are followed by afterglows, which are visible at X-ray and optical wavelengths for a few hours or days.

Kulkarni and Bloom presented evidence in the Sept. 29 issue of the journal Nature suggesting that the longer-lasting gamma-ray bursts are produced by supernovae, explosions caused in these cases by collapsing stars 20 to 30 times more massive than the sun. Their work was based on the study of a gamma-ray burst that took place on March 26, 1998.

Kulkarni and Bloom’s work earlier this year convinced Lamb that the longer gamma-ray bursts originate from the death throes of massive stars. Lamb’s theoretical research group set to work on the problem. Within weeks, Reichart had confirmed the Caltech finding with even more rigorous data from a burst detected on Feb. 28, 1997. He reported his results in a recent issue of the Astrophysical Journal. The burst’s measurements were precisely consistent with a model predicting that gamma-ray bursts are produced following the collapse of a massive star, resulting in the formation of a black hole.

Astronomers previously regarded quasars as the most distant objects in the universe. The most distant quasars, believed to be the seeds of young galaxies, formed at a redshift of 5. Redshift is a measure of celestial distance. The higher the redshift, the more distant the object.

A redshift of 5 corresponds to a distance of 13 billion light years, when the universe was 7 percent of its current age. But Lamb and Reichart have calculated that gamma-ray bursts theoretically should be visible out to a redshift of 20, when the universe was only a hundred million years old, or approximately 1 percent of its current age. Beyond a redshift of 20, astronomers believe no stars were forming or collapsing to produce gamma-ray bursts.

The optical afterglows that follow the bursts contain vital information about the early universe.

“By looking at the spectrum of these very bright burst afterglows, we can look at the large-scale structure of the universe back to redshifts of 10 or 20,” Lamb explained. Various cosmological theories make specific predictions about how structure in the universe should first form and evolve, as well as what it would look like. “There’s been no conceivable way we might check that formation and evolution. Gamma-ray bursts, if they’re produced by massive stars, are going to be the probe to do it.”

Reichart said he anticipates at least a mini-revolution in the field of gamma-ray bursts following a successful launch of HETE-2 in January. Much the same thing happened when BeppoSax, an Italian-Dutch satellite, discovered afterglows in 1997. But BeppoSax was not designed to look for burst afterglows, and scientists must wait eight hours or more to make follow-up observations. Even then, they can do so only if they have access to powerful telescopes.

HETE-2, by contrast, will allow scientists using modest-sized optical telescopes to view an afterglow just five or 10 seconds after the burst occurs.

“All sorts of things are going to happen, things that we can’t even possibly predict,” Reichart said.