gamma-ray burst

gamma-ray burst An intense burst of gamma‐ray radiation, lasting from a few milliseconds to a few tens of minutes. Gamma‐ray bursts were initially detected in the late 1960s by US Vela satellites designed to monitor nuclear explosions, and have since been studied in detail by satellites such as the Compton Gamma Ray Observatory (CGRO), BeppoSAX, the High‐Energy Transient Explorer‐2, the International Gamma‐Ray Astrophysics Laboratory (INTEGRAL), and Swift. CGRO showed that the bursts are distributed uniformly over the sky, indicating that they originate in distant galaxies and fall into two types, short and long, suggesting two distinct mechanisms. Short bursts last from a few milliseconds up to 2 seconds; these are believed to be caused by the merger of two compact objects such as black holes, neutron stars, or possibly white dwarfs. Long bursts last from a few seconds to over a thousand seconds and are believed to be caused by massive stars exploding as supernovae, with the collapsing core of the star forming a rapidly rotating black hole. In both scenarios, an outflow of gas creates shells that collide internally at close to the speed of light, producing the burst of gamma rays. The combined shells expand and collide with the surrounding gas and dust of the interstellar medium, heating it so that it begins to emit afterglow radiation at X‐ray wavelengths. As the gas cools the afterglow becomes visible at optical, infrared, and radio wavelengths, and this emission may remain detectable for days to years. See also SOFT GAMMA‐RAY REPEATER.