Farthest Gamma-Ray Burst
by Paul Price and Bob Joseph
The Swift Satellite is designed to turn in less than a minute to point
at gamma-ray bursts as they go off. Image courtesy of http://swift.sonoma.edu/images/multimedia/images/sc/swift.jpg .
Gamma-ray bursts (GRBs) are the most luminous celestial events we know
of, so they permit us to see more deeply into the distant Universe than
is possible with any other phenomenon. Astronomers from Japan and IfA recently
used telescopes on Mauna Kea and Haleakala to measure the distance
to the most remote GRB ever detected.
Astronomers believe that GRBs are the death shrieks of a massive star
that accompany a supernova explosion. GRBs last from a few thousandths
of a second to a few minutes, but they leave an "afterglow" that
lasts for several hours. Where there are dying massive stars, there are
living stars as well, so GRBs give astronomers the opportunity to study
stellar birth and death in the most distant Universe in a manner only dreamed
about before they were discovered.
The Vela satellites, launched in the late 1960s to monitor compliance
with the nuclear test ban treaty, serendipitously discovered these heavenly
flashes of gamma-rays, which are the highest-energy particles in the electromagnetic
spectrum. Because the Vela data were classified, the first scientific paper
announcing the discovery of GRBs did not appear until 1973.
The Vela data enabled scientists to make only very crude determinations
of the positions of the GRBs, and so it was not even possible to determine
whether they came from our own Milky Way galaxy or from other, more distant,
galaxies. Without knowing the distance, they were unable figure out what
kinds of physical processes produced them because they did not know the
intensity of gamma-ray radiation they produced.
In 1991, NASA launched the Compton Gamma-Ray Observatory, which gave improved,
but still crude, positions for GRBs. The information collected by this
satellite showed that GRBs are spread randomly over the entire sky. This
means they are not coming from within the Milky Way, because in that case
they would be located only in the plane of our galaxy, just like the stars
that make up the Milky Way. Scientists concluded that GRBs come from deep
space and are probably associated with other galaxies.
Astronomers confirmed this in 1997 when they detected a GRB's afterglow
that they measured in the visible and radio parts of the spectrum. Since
then, over 50 GRBs have been identified, usually in galaxies around 6 billion
light-years away. In April, NASA launched the Swift Gamma-Ray Burst satellite,
which is able to measure the positions of GRBs more precisely than
its predecessors. On the morning of September 4, Swift identified a new
GRB, 050904. Astronomers at observatories around the world, including IfA's
Lennox Cowie and Paul Price, rushed to observe it. Working with astronomers
Yuzuru Yoshii and Takeo Minezaki at the University of Tokyo and the Japanese
MAGNUM telescope on Haleakala, we took images of this GRB that enabled
us to estimate the distance to the explosion from the visible and infrared
radiation of its afterglow. We concluded that this GRB was much more distant
than any other GRB observed up to now.
Nobuyuki Kawai from the Tokyo Institute of Technology led a team that
used the 8.2-meter Subaru Telescope on Mauna Kea to make a precise measurement
of distance to the explosion: 12.8 billion light-years. This is the most
distant explosion astronomers have ever seen. There are fewer than fifty
other known objects at such a great distance from Earth, and the farthest
is only a mere 50 million light-years (or 0.4 percent) more distant.
We hope that Swift will soon find a GRB even farther away, so that we
can use it to study the Universe beyond what anyone has ever seen.
|Subaru FOCAS (Faint Object Camera
and Spectrograph) images of the afterglow of the gamma-ray
burst GRB 050904 in two different filters (8000 angstrom
Ic and 9000 angstrom Z' band). The different appearance
hints at the great distance of the object. © Subaru Telescope