The first real breakthrough came in the early 1990s with the launch of the Compton Gamma Ray Observatory. In particular, the Burst and Transient Source Experiment (BATSE) obtained positions for several thousand GRBs to show that they were distributed uniformly over the entire sky. This was the first suggestive evidence that GRBs were cosmological entities (objects of the distant Universe) and not Galactic in nature (in which case they would have been concentrated in the Galactic plane or towards the Galactic centre). The problem then was that although BATSE could detect GRBs, it couldn’t position them accurately enough on the sky to allow for follow-up observations at other wavelengths.
This problem was alleviated with the 1996 launch of the Italian-Dutch satellite, BeppoSAX, which could pinpoint a GRB to within several arcminutes. With error circles this small, other satellites and ground-based telescopes were able to search for afterglows (lower energy emission predicted to accompany GRBs) at other wavelengths.
In 1997 we finally obtained conclusive proof that GRBs occupy the distant reaches of the Universe. A spectrum of the optical transient associated with the GRB named GRB970508 contained an absorption spectrum of a galaxy located along the line of sight at a cosmological redshift of 0.835. This meant that the GRB itself was even more distant, placing GRBs at cosmological distances (billions of light years away).
Since the late 1990s, the detection of optical transients and the afterglows associated with GRBs has become almost standard practice. One of the primary mission goals of several current satellites (e.g. HETE-2, SWIFT, INTEGRAL, RXTE, ULYSSES) is to detect and triangulate GRB positions quickly and relay this information via the Gamma Ray Burst Coordinates Network to other satellites and ground-based observatories for follow-up observations at other wavelengths. With this network, it is now possible to observe a GRB at different wavelengths within 1 minute of the original detection of the burst by a satellite. Since GRBs are fleeting transient objects, such rapid response is vital to unravelling the nature of these powerful explosions.