Gamma-ray bursts (
GRBs) are flashes of
gamma rays associated with 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. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (
X-ray,
ultraviolet,
optical,
infrared,
microwave and
radio).