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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 FOCUS image
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