Gamma-Ray Bursts: The Most Powerful Cosmic Explosions

Abstract.

The field of Gamma Ray Burst (GRB) research is one in which discovery by serendipity plays an important role. Serendipity in general means: one searches for something but finds something else, which often is more interesting. Generally in astrophysics this comes about because one has a new instrument that can measure some physical aspect at least an order of magnitude better than was possible before. For example, the new instrument has an order of magnitude better sensitivity, or spectral resolution or angular resolution. The discovery of the GRBs was itself a classical example of serendipity. They were discovered in 1967 with the US military Vela satellites, which had been built to monitor whether countries were keeping to the Nuclear Test Ban Treaty that had been signed earlier in the sixties. To this end the Vela satellites were built to be sensitive to the gamma ray flash of nuclear explosions in the Earth’s atmosphere or in space. To check for possible radioactivity produced by explosions on the backside of the Moon, the Vela satellites had very wide orbits extending halfway to the Moon. There were always several of them orbiting the Earth at any given time. In 1967 they detected gamma ray flashes of much longer duration than expected from a nuclear explosion, and from the differences in arrival time of these flashes in the different Vela satellites the Los Alamos scientists could roughly determine the direction from which the flashes came. It turned out that they did not come from Earth but from the sky. The discoverers were so surprised by this result that they studied the bursts for a long time, until they were absolutely sure that this was a real phenomenon. In 1973 they presented their discovery to an astrophysical audience [33], which caused a sensation. Theorists produced dozens of theories about their possible origin, ranging from comets colliding with neutron stars to nuclear wars of extraterrestrial civilizations. For 30 years the places of origin of the GRBs remained a mystery, as none could be identified with a known object. It was not even known whether they came from nearby, in our own galaxy, or from very far away, at cosmological distances. Only in 1997, thanks to the Wide-Field imaging hard X-ray cameras aboard the Italian-Dutch BeppoSAX satellite it became possible to swiftly determine their positions on the sky with sufficient accuracy that identification with known objects could be made. It was found that GRBs exhibit afterglows at optical and soft X-ray wavelengths, which persist for a few days [57,10]. This allowed localizing their places of origin in distant galaxies with redshifts between 0.3 and 4.5, corresponding to “distances” (look-back times) of between 4 and 11 billion lightyears. From this it became clear that GRBs are the most powerful photon-emitters in the Universe with fluxes that for a short time rival the total photon emission of the observable Universe. They are the most powerful explosions in the Universe since the Big Bang, sometimes reaching an intrinsic optical brightness a million times that of a supernova. It has since been found that GRBs occur outside the nuclei of their host galaxies, and that some GRBs coincide with peculiar and very energetic supernovae. This indicates that they are stellar phenomena, presumably related to the death of very massive stars. The host galaxies in most cases show evidence of a high formation rate of massive stars, which independently suggests that GRBs are related to the evolution of these short-lived massive stars.