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.
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
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.
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)|
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.