|The Latest on the Structure and Evolution of Our Universe
Vol 9. No. 5
24 May 2005
NASA Scientists Catch Unique Gamma-Ray Burst
The region where the GRB was detected.
The "XRT Error Circle" identifies the location of the GRB. Four
possible optical candidates are shown, but these were later
discounted. (Credit: S. Bradley Cenko, LRIS/Keck)
After 30 years, they finally caught one. Using the Swift satellite,
scientists for the first time detected and pinned down the location
of a so-called "short" gamma-ray burst, one lasting only 50
A gamma-ray burst (GRB) marks the birth of a black hole. In recent years
astronomers have discussed two possible causes of GRBs - one is the
collapse of a supermassive star in an event called a hypernova. The
other is a collision of two older black holes, or two neutron stars.
In addition, astronomers have noticed that there appear to be two
types of GRBs. Most GRBs last a few seconds or some tens of
seconds. Satellites such as Swift, along with follow-up observations
by ground-based telescopes, have shown these to be in young,
distant galaxies filled with young, massive stars. Such massive stars
are prime candidates for the hypernova model. Other GRBs last a
second or less. These have had remained elusive until Swift
detected this one, autonomously locked onto a location, and focused its
on-board telescopes in less than a minute to capture the burst
"Seeing the afterglow from a 'short' gamma-ray burst was a major goal
for Swift, and we hit it just a few months after launch," said Dr.
Neil Gehrels, Swift project scientist at NASA Goddard Space Flight
Center in Greenbelt, Md. "Now, for the first time, we have real data
to figure out what these things are."
This burst is called GRB 050509B, being the second burst
detected on May 9, 2005. Swift's X-ray Telescope (XRT) detected a
weak afterglow that faded away after about five minutes. Swift's
Telescope did not see an afterglow. Ground-based telescopes have not
yet definitely detected an afterglow either. In contrast, afterglows
from long bursts linger from days to weeks.
Most importantly, Swift was able to determine the position of this
burst to within about 10 arc seconds. GRB 050509B appears to have
occurred near a giant elliptical galaxy that has
old stars and is relatively nearby (about 2.7 billion light years
away). These galaxies are often surrounded by globular clusters,
which are dense clusters of old stars. This would be consistent
with the theory that short bursts come from collisions of older,
evolved neutron stars or black holes.
"Everything about this gamma-ray burst so far supports the merger
theory," said Dr. Steinn Sigurdsson, a theorist at Penn State and
expert on black hole mergers.
"We are combing the region around the burst with the Keck Telescope
for clues about this burst or its host galaxy," said Caltech Prof.
Shri Kulkarni, a gamma-ray burst expert. "What we are seeing so far
is what proponents of the merger theory have been saying all along.
Such an evanescent afterglow has been expected in the most popular
model for short hard bursts to date."
Observations by S. Bradley Cenko at the 10-m Keck I Telescope
identified possible optical counterparts to the burst. However,
subsequent observations showed that none of them varied in intensity,
as would be expected. Further follow-up
observations are planned with an armada of telescopes, both
NASA's Hubble Space Telescope and Chandra X-ray Observatory.