Gamma Ray Bursts (or GRB's) are the most luminous electromagnetic events occurring in the universe since the Big Bang.
Gamma Ray Bursts (GRB) are named after the date on which they are discovered: the first two digits being the year, followed by the two-digit month and two-digit day. If two or more GRBs occur on a given day, the name is appended with a letter 'A' for the first burst identified, 'B' for the second and so on.
Most observed GRBs appear to be emissions caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, the leading theory being the merger of neutron stars orbiting in a binary system. All observed GRBs have originated from outside the Milky Way galaxy, though a related class of phenomena, soft gamma repeater flares, are associated with galactic magnetars. The sources of most GRBs have been billions of light years away.
Gamma-ray bursts were discovered in the late 1960s by the U.S. Vela nuclear test detection satellites. The Velas were built to detect gamma radiation pulses emitted by nuclear weapon tests in space. The United States suspected that the USSR might attempt to conduct secret nuclear tests after signing the Nuclear Test Ban Treaty in 1963. While most satellites orbited at about 500 miles above Earth's surface, the Vela satellites orbited at an altitude of 65,000 miles. At this height, the satellites orbited above the Van Allen radiation belt, which reduced the noise in the sensors. The extra height also meant that the satellites could detect explosions behind the moon, a location where the United States government suspected the Soviet Union would try to conceal nuclear weapon tests.
The Vela system generally had four satellites operational at any given time such that a gamma-ray signal could be detected at multiple locations. This made it possible to localize the source of the signal to a relatively compact region of space. While these characteristics were incorporated into the Vela system to improve the detection of nuclear weapons, these same characteristics were what made the satellites capable of detecting gamma-ray bursts.
On July 2 1967 the Vela 4 and Vela 3 satellites detected a flash of gamma radiation that were unlike any known nuclear weapons signatures. Nuclear bombs produce a very brief, intense burst of gamma rays less than one millionth of a second. The radiation then steadily fades as the unstable nuclei decay. The signal detected by the Vela satellites had neither the intense initial flash nor the gradual fading, but instead there were two distinct peaks in the light curve.
Shortly after the discovery of gamma-ray bursts it was decided that in order to determine what caused them they would have to be paired with astronomical objects at other wavelengths. This method would require far more accurate positions of several gamma-ray bursts than the Vela system could provide. Greater accuracy required the detectors to be spaced farther apart. Instead of launching satellites only into Earth's orbit, it was deemed necessary to spread the detectors throughout the solar system.
By the end of 1978 the first Inter-Planetary Network (IPN) had been completed which consisted of the original Vela satellites and 5 new space probes within the solar system. When the system was active a gamma ray burst could be pin pointed to within a few arc minutes, however even with the most powerful telescopes, nothing of interest could be found within the area.
The Compton Gamma Ray Observatory was launched in 1991 together with its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data indicating that GRBs are isotropic (not biased towards any particular direction in space, such as toward the galactic plane or the galactic centre). Because the Milky Way galaxy has a very flat structure, if gamma-ray bursts were to originate from within the Milky Way, they would not be distributed isotropically across the sky, but instead concentrated in the plane of the Milky Way. Although the brightness of the flashes suggested that the bursts had to be originating within the Milky Way, the distribution indicated otherwise.
A nearby gamma-ray burst could possibly cause mass extinctions on Earth. The short duration of a gamma-ray burst would limit the immediate damage to life. However a nearby burst might alter atmospheric chemistry by reducing the ozone layer and generating acidic nitrogen oxides, ultimately causing severe damage to the biosphere.
On analysis of the distribution of GRBs, it has been found that metal-deficient galaxies are the most likely to contain sources of highly energetic, long GRBs. Because the Milky Way is presumed too metal-rich to host a long GRB, and the fact that no GRBs have emitted in the out galaxy since the Earth formed, it is most unlikely that a nearby GRB has caused mass extinction events on Earth in the past.
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