# Gamma-ray Bursts: 50 Years and Counting!
## 2 The \"Dark\" Era (1973(67)-1991)
A clear prediction of all these models was that, given the galactic nature of the phenomena, the spatial distribution of GRBs would be centered around the Galactic Equator. The hope was, therefore, that by placing more sensitive satellites with superior localization capabilities into orbit, a distribution along the plane of the Milky Way would be observed. We now note that a subclass of GRBs is definitively of galactic origin: Soft Gamma-ray Repeaters (SGRs), objects that emit repeated flashes of gamma and X-rays at irregular intervals. It is hypothesized that they could be a type of magnetar or neutron star surrounded by fossil disks [9]. Due to the large distances, extragalactic models in general had very high energy demands (with energies up to \(10^{51}\)-\(10^{53}\) erg). This, in addition to implying a particularly low rate of events (\(10^{-6}\)-\(10^{-5}\) events per year in a galaxy with the luminosity of the Milky Way), has led to the proposal of exotic phenomena and/or objects (collapsing white dwarfs, stars accreting onto AGNs, white holes, accretion onto black holes in binary mergers or collapsing stars) among the possible origins of GRBs. The first extragalactic model of GRBs was probably the one proposed by [PERSON] and [PERSON] [10], who proposed that GRBs are produced in the collapse of the cores of active galaxies. As early as 1975, [PERSON] recognized that the electron-positron pair production condition is a limiting factor for the achievable GRB luminosity (the so-called compactness problem) [11]. He noted that this condition posed fewer challenges for galactic models compared to extragalactic ones and pointed out that relativistic motion had the potential to mitigate the compactness problem by enlarging the emission size. In 1978, using a more elaborate version of the two-photon pair production condition proposed by [PERSON] to set general constraints on the luminosities of GRBs, [PERSON] and [PERSON] proposed, for the first time, a fireball model for modeling GRB emissions [12]. In 1989, [PERSON] and colleagues conducted an extensive examination of the neutron-star-neutron-star (NS-NS) merger model, presenting a detailed analysis of its potential relevance to gamma-ray bursts (GRBs). Their work introduced the notion that mergers of compact objects (either NS-NS, NS-black hole, or BH-BH) could serve as the progenitors for a specific subclass of observed GRBs [13]. In the paper, it was also suggested that these systems are important multi-messenger emitting sources, as, beyond gravitational waves, binary neutron star mergers are important sources of neutrino emissions, as well as fundamental heavy-element factories through the rapid neutron capture process (the r-process) of neutron-rich material ejected from the merger. Ref. [13] laid the foundation for the standard paradigm of the modern short-GRB models. This era can be summarized as follows:  
* GRBs were discovered in the 1960s by the Vela military satellites and later observed by other missions. * The spatial distribution and total energy of GRBs remained unknown due to low instrumental capabilities and a low number of events. * Galactic models suggested that GRBs were related to processes like supernovae, accretion onto compact objects and magnetic reconnection in magnetars. * Galactic models predicted GRBs to be distributed on the galactic plane. * Extragalactic models had high energy demands, and possible origins included collapsing white dwarfs, active galaxy cores, and neutron star mergers. * The compactness problem and relativistic motion were already considered in these models. * The relativistic fireball model was introduced to explain GRB emissions.