Published online by Cambridge University Press: 05 December 2012
Introduction
For 40 years theorists have struggled to understand gamma-ray bursts (GRBs), not only where they are and the systematics of their observed properties, but what they are and how they operate. These broad questions of origin are often referred to as the problem of the “central engine.” So far, this prime mover remains hidden from direct view, and will remain so until neutrino or gravitational-wave signatures are detected. As discussed elsewhere in this volume, there is compelling evidence that all GRBs require the processing of some small amount of matter into a very exotic state, probably not paralleled elsewhere in the modern Universe. This matter is characterized by an enormous ratio of thermal or magnetic energy to mass, and the large energy-loading drives anisotropic, relativistic outflows. The burst itself is made far away from this central source, outside the star that would otherwise obscure it, by processes that are still being debated (Chapters 7 and 8). The flow of energy is modulated by passing through the star, which also explodes as a supernova, and this modulation further obscures details of the central engine.
The study of GRBs experienced spectacular growth after 1997 when the first cosmological counterparts were localized (Chapter 4), and with that growth in data came increased diversity. Still, it is customary to segregate GRBs into “long-soft” (LSBs) and “short-hard” (SHBs) categories (Kouveliotou et al. 1993), though the distinction is not always clear (Chapters 3 and 5; Section 10.5.9).
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