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Gamma-Ray Bursts : انفجار پرتو گاما
The serendipitous discovery of cosmic gamma-ray bursts (GRBs) was announced in 1973, based on the data of the military Vela spacecraft, whose purpose was to monitor the limited nuclear test ban treaty. However, it was not until the late 1990s that a clear idea of their origin was obtained. A GRB is a short-duration pulse of gamma radiation (<20 keV to energies up to tens of GeV). With instruments of sufficient sensitivity, about one burst per day can be observed. GRB durations range from 10 ms to 1000 s or more, but an average is about 10 s. For this short time, they may be the brightest objects in the gamma-ray sky. An example is shown in figure 1. Each burst comes from a unique, random direction; no two events have ever been detected from exactly the same direction. The distribution of burst sources on the sky is completely isotropic (figure 2). These properties, which are quite unlike those of typical gamma-ray sources (see gamma ray astronomy) explain, in part, why progress in understanding the origin of bursts has been slow: Gamma-ray telescopes tend to have small fields of view and provide relatively coarse location information, making it impractical to monitor the entire sky and provide precise directional information with a single detector. The main detection techniques to date have been interplanetary networks and arrays of large-area detectors such as the Burst and Transient Source Experiment aboard the Compton Gamma-Ray Observatory. However, the inherent delays in the former, and the relatively coarse localizations provided by the latter, hampered efforts to detect burst sources at other wavelengths.
A breakthrough was achieved in 1997 by the Italian–Dutch BeppoSAX spacecraft, which carried both a wide-field x-ray camera and narrow-field x-ray telescopes. The wide-field camera made it possible to detect and locate bursts in the 2–10 keV energy range with a precision of several arcminutes. The spacecraft could then be slewed within hours to view the position of the burst source with the narrow-field telescopes. It was discovered that, long after the burst had disappeared in gamma-rays, a fading x-ray afterglow.
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