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Chasing the Explosion

Not until astronomers were able to make afterglow observations could they develop a working hypothesis on what caused gamma-ray bursts. And while the Compton Gamma Ray Observatory's Burst And Transient Source Experiment (BATSE) detector catalogued 2,704 GRBs during the observatory's nine year lifetime (1991 - 2000), it was not equipped to make afterglow observations. Furthermore, it had not been possible to get either a ground or space-based telescope look up quickly enough to a spot where a GRB had been detected.

As a result, the first afterglow observation did not come until the BeppoSAX satellite. BeppoSAX was an Italian satellite which was equipped with both a gamma ray and an X-ray detector. It spotted the X-ray afterglow signature associated with the gamma-ray burst on February 28, 1997 (dubbed GRB 970228 using the standard naming convention). Up until that time, it simply wasn't possibly to get either a ground or space-based telescope to look quickly enough at a spot where a GRB had been detected.

GRB 970228
An X-ray image of the gamma-ray burst GRB 970228, obtained by the team of Italian and Dutch scientists at 5:00 AM on Friday 28th February, 1997, using the BeppoSAX satellite. (Larger image)
Today a worldwide network called the Gamma-ray Burst Coordinates Network (GCN) coordinates space-based observations and ground-based follow-through observations of GRB afterglow. NASA satellites include the High Energy Transient Explorer (HETE) operated by the Massachusetts Institute of Technology and the Rossi X-ray Timing Explorer (RXTE). The European Space Agency operates Integral, a new gamma-ray mission launched in 2002. And there is the Interplanetary Gamma-Ray Burst Timing Network (IPN), which consists of a group of space probes with gamma-ray detectors at different locations in the Solar System.

By timing the arrival of gamma-ray photons at each satellite, the location of the burst can be "triangulated." The GCN sends out automatic notices by email to astronomers worldwide, enabling both professional and amateur astronomers to make follow-up afterglow observations.

GRB-Supernova Link: The Proof

Initial evidence that GRBs were linked to supernova came with the study of GRB 980425 in 1998. That burst was tentatively linked to a supernova called SN 1998bw located in a distant galaxy.

Definitive proof of the supernova link, at least in the case of those GRBs with an afterglow, came on March 29, 2003, when a relatively nearby burst, GRB 030329 produced an afterglow whose optical spectrum was nearly identical to a supernova. X-ray observations also showed a signature associated with oxygen heated to high temperatures. Such a light pattern occurs when the supernova blast wave excites oxygen atoms in the vicinity of the star. These observations constituted the "smoking gun," providing even more solid evidence than GRB 980425.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

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