A new way to find distance of gamma-ray bursts
Posted: January 12, 2002

Astronomers at Rice University in Houston have discovered that the rate at which a gamma-ray burst cools might be used to calculate the distance of that burst.

Their findings are being presented this week at the American Astronomical Society meeting in Washington, D.C. The researchers believe that this additional technique will enable scientists to learn more about the evolution of the early universe.

Gamma-ray bursts are transient, short flashes of gamma rays that occur randomly in the sky every day. The gamma rays themselves cannot be seen by human eyes, but astronomers' instruments in orbit around Earth can detect them. Since 1997, scientists have known that these bursts represent gigantic explosions likely associated with the death of massive stars at a distance of about 10 billion light years in the early part of the universe. By determining the distance of gamma-ray bursts, astronomers hope to trace the formation of massive stars and the structure and evolution of the early universe.

About 3,000 gamma-ray bursts have been recorded, mostly during the 1990s, but astronomers know the actual distance to only a very few bursts. In recent years, two methods have been proposed for indirectly calculating the distance from the available data. Edison Liang and Dan Kocevski at Rice University, collaborating with Brad Schaefer at The University of Texas at Austin, have come up with a third.

"It's well-known that gamma-ray bursts start at high energy and evolve to lower energies," said Liang, a professor of physics and astronomy at Rice. Gamma-ray spectrometers convey this shift in energy through changes in color, going from blue (gamma rays with high energy) to red (lower energy).

"We examined 16 gamma-ray bursts and found that the apparent rate at which the burst is cooling off appears to be directly related to the distance of the burst, provided that the rate is measured not in terms of time, but in terms of the total number of gamma rays emitted since the beginning of the pulse," Liang said.

But this technique works only on gamma-ray bursts that have separable pulses, or peaks, of intensity. Bursts that are "chaotic" have multiple peaks, or spikes, of energy. The combination of data from multiple overlapping gamma-ray pulses makes it difficult to estimate the true cooling rate of the highest peak, according to Kocevski, a graduate student at Rice. "You tend to underestimate the cooling rate when observing bursts with multiple peaks," he said.

Since the majority of gamma-ray bursts are of the chaotic variety, Liang and Kocevski are now trying to develop methods to separate the color of overlapping pulses from within the chaotic bursts to determine the true cooling rate. Using software the Rice group developed to measure the cooling of bursts, the researchers are hopeful that they will be able to apply their technique for calculating a gamma-ray burst's distance to chaotic bursts. This would expand the database of knowledge from which deductions about the formation of the early universe can be made and provide new insights into the physical mechanisms of these enigmatic explosions.

"It's very labor-intensive and tedious, but we have high hopes it will work eventually," Liang said.