# Gamma-ray Bursts: 50 Years and Counting!
## 2 The \"Dark\" Era (1973(67)-1991)  
The discovery of GRBs was made by the US military Vela satellites [1] in the late 1960s. These were a satellite constellation launched in order to monitor Soviet compliance with the Partial Test Ban Treaty of 1963, which prohibited all test detonations of nuclear weapons, except for those conducted underground. In order to look for traces of nuclear tests in space, the Vela satellites were equipped with scintillation X-ray detectors, sensitive between 3 and 12 keV, and highly sensitive CsI gamma-ray detectors, sensitive between 150 and 750 keV. The satellites were launched in pairs, in a common circular orbit at an altitude of 118,000 km, in order to be well above the Van Allen radiation belts, drastically reducing the noise in the sensors and allowing some localization of the signal through time triangulation techniques. At 14:19 UTC on 2 July 1967, the Vela 3 and Vela 4 satellites recorded an unprecedented gamma-radiation burst distinct from any previously identified astrophysical source. Despite the limited temporal resolution of the Vela satellites, GRB670702 (as it would be called with the modern-day naming convention) exhibited fundamental characteristics typical of GRBs (such as a duration of approximately 10 s, a two-pulse light curve and a peak emission around the MeV energy band), very different from the properties (duration, spectrum, variability) expected for a nuclear test in space (a short-duration, hard, nonstructured X-rayburst). Although the data were never classified (the emission was not of nuclear origin), they were only published in 1973 [1], after a total of 16 such signals had been observed. Around the same time, other instruments reported the observation of GRB signals, such as the American satellite IMP-6 [2] and the Soviet space satellite Konus/Venera [3]. The low number of signals made it impossible to estimate the spatial distribution of the events, and the lack of a method to determine the distance of their sources meant that the total energy and the location remained unknown. The poor localization capability of gamma-ray detectors made it very difficult to discover the electromagnetic counterparts of GRBs at lower frequencies. Still, a lot of information was gathered. It was clear that these signals, not thermal in origin, were not coming from Earth and not even from the Solar System plane. Spectral lines, not confirmed by subsequent missions, were reported with low significance in some GRBs detected with the Soviet Konus/Venera instruments [4] and with the Japanese Ginga satellite [5; 6]. With so few constraints, theorists came up with all kinds of models based on a wide range of physics, with energy requirements varying over 20 orders of magnitude. By 1992, over 100 models existed [7], some proposed even before the official publication of the Vela results. The proposed models can be broadly divided into three categories: Earth and Solar System (for example, Terrestrial Gamma-ray Flashes, gamma rays produced by lightning at high altitudes in Earth's atmosphere), galactic, and cosmological models. Galactic models, favored by the lower energies required in the processes, posited that the GRBs were produced either by the accretion/fall of material onto compact objects (supernova fallbacks, accretion from a binary companion onto white dwarfs or neutron stars, later observed as X-ray Novae) or through magnetic reconnection/quakes in the crusts of magnetars (neutron stars with magnetic fields reaching \(10^{15}\) G). The first models came out even before the publication of the Vela results, and as early as 1968, [PERSON] proposed a model of GRBs as bremsstrahlung and inverse Compton emissions from Type II supernova shocks [8]. According to the current standard paradigm, long GRBs are indeed associated with supernovae, but only with one special type, namely, broad-line Type Ic, not Type II as envisaged by [PERSON].