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Gamma Ray Bursts

Gamma-ray bursts (GRBs) have remained to be among the most mysterious objects in modern astrophysics, since their discovery about three decades ago. They are gamma-ray flashes in the sky, releasing tremendous amount of energy in as little time as a few seconds. Observations have established that most, if not all, GRBs are of extragalactic origin and that they seem to be associated with regions of active star formation in other galaxies. However, the physical origin of the central engine in a GRB is still being debated. Numerous scenarios have been proposed over the recent years. One of the leading models postulates that GRBs are produced in the ``death'' of very massive stars as they collapse to form black holes (in the form of a ``collapsar'' or ``supranova''). This is supported by the observed association between some GRBs and supernova explosions. In these models, the prompt gamma-ray emission is thought to originate in the highly relativistic jets associated with a rapidly spinning black hole and the jets are roughly along the line of sight of the observer. If this is proven, GRBs should bear some resemblance to blazars and would be potential VHE gamma-ray emitters. It is, therefore, not surprising that a number of GRBs were detected by EGRET (at MeV-GeV energies). However, there has been no positive detection of any GRB at TeV energies, despite of extensive searches. Like blazars, the very high energy gamma ray emission from GRBs could be of leptonic and/or hadronic origin, depending on the composition of the jets. It has been speculated that GRBs might be sources of ultra high energy cosmic rays (UHECRs). The observations of GRBs at GeV-TeV energies might shed significant light on this long-standing issue.