A gamma-ray burst is a short, intense flash of gamma rays that can last from a few milliseconds to about a hundred seconds. This is followed by an afterglow of longer wavelength radiation that can last for weeks or even years. Until now, however, there has always been a delay between detection of the burst and the first observation of the afterglow.

Fox and co-workers used the High Energy Transient Explorer II (HETE II), which can transmit accurate gamma-ray burst positions to Earth in real time. On 4 October 2002 HETE II sent a signal just 49 seconds after a burst - called GRB 021004 – was located. Among the first telescopes to respond were the Automated Response Telescope in Japan and 48-inch Palomar Oschin Telescope in California.

Other telescopes across the globe then continued taking images for several weeks until the afterglow finally disappeared.

The researchers found that the early afterglow decayed much more slowly than predicted, which means that gamma-ray bursts must be more powerful than previously thought. These findings support the “collapsar” model in which the core of a massive star collapses into a spinning black hole. Moreover, further analysis of the X-ray and optical afterglow showed unusual and hitherto unseen energy variations in the spectrum.