ASTRONOMERS TOUT GAMMA-RAY BURSTS AS PROBES INTO EARLY UNIVERSE

WAIMEA, Hawaii (October 22nd, 1999) 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 University of Chicago astronomers.

Don Lamb Jr., a Professor in Astronomy & Astrophysics at Chicago, will describe the cosmological implications of gamma-ray bursts at 10:30 a.m. 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 University of 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, 2000, 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, according to Lamb. “We’ll be able to trace, perhaps, the very earliest beginnings of gravitationally bound structures and how they merged to form honest-to-goodness galaxies.”

The origin of these bursts have 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 that are visible for a few hours or days at X-ray and optical wavelengths.
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 a 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 that took place 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 distance 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 distances. 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 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, corresponding to 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 Chicago scientists took the seven gamma-ray bursts whose distances and brightness are well-documented, and calculated that the HETE-2 and Swift satellites could still detect them at far greater distances.

“These gamma-ray bursts are so bright that they can be detected out to redshifts that boggle the mind,” Lamb said. “The Swift mission could see bursts out to a redshift of 70, although I doubt if such bursts exist.” The orbiting Chandra Observatory could detect their X-ray afterglows out to similar redshifts, he said.

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 should first form, evolve and what it would look like. “There’s been no conceivable way we might check that. 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 to view the afterglow just five or 10 seconds after the burst occurs using modest-sized optical telescopes.

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