Gamma-ray bursts (GRBs) are sudden, intense flashes of gamma rays that, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. They are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last 3 or 4 years, including the discovery of slowly fading x-ray, optical, and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and evidence showing that many GRBs are associated with star-forming regions and possibly supernovae. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock modeL. These advances have opened new vistas and questions on the nature of the central engine, the identity of their progenitors, the effects of the environment, and their possible gravitational wave, cosmic ray, and neutrino luminosity. The debates on these issues indicate that GRBs remain among the most mysterious puzzles in astrophysics.
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