Gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several hours. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).
The positions are estimated by exploiting the time delays between the arrival times of the photons of different detectors, as shown in the figure. To measure short delays, the detectors must have a good temporal resolution, and their relative position must be known with good accuracy. The accuracy in the determination of the position of a transient event will depend on the number of detectors.