Astronomers at Rice University find another way to measure distance of gamma-ray bursts

CONTACT: B.J. Almond
PHONE:
(713) 348-6770
EMAIL: balmond@rice.edu

ASTRONOMERS AT
RICE UNIVERSITY FIND ANOTHER WAY
TO MEASURE DISTANCE OF GAMMA-RAY
BURSTS

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 today 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.
Images of gamma-ray bursts used for this study, which was
supported by NASA, can be found at <www.kocevski.com/research>.