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SGR 1806-20 is a magnetar, a particular type of neutron star. It has been identified as a soft gamma repeater. SGR 1806-20 is located about 14.5 kiloparsecs (50,000 light-years) from Earth on the far side of our Milky Way galaxy in the constellation of Sagittarius. It has a diameter of no more than 20 kilometres (12 miles) and rotates on its axis every 7.5 seconds (30,000 km/h rotation speed at the surface). As of 2007, SGR 1806-20 is the most magnetic object ever perceived by mankind, with a magnetic field over 1015 gauss in intensity (compared to the Sun's 1-5 gauss).
On December 27
, the radiation from an explosion on the surface of SGR 1806-20 reached Earth
. In terms of gamma rays the burst was brighter than a full moon and had an absolute magnitude
of around −29. It was the brightest event known to have been sighted on this planet from an origin outside our solar system. The gamma rays struck the ionosphere
and created more ionization which briefly expanded the ionosphere. The magnetar released more energy in one-tenth of a second (1.3×1039 J
) than our sun has released in 100,000 years (4×1026 W
s = 1.3×1039
J). Such a burst is thought to be the largest explosion observed by humans in the galaxy since the SN 1604
supernova observed by Johannes Kepler
A similar blast within 3 parsecs (10 light years) of earth would destroy the ozone layer and would be similar to a 12 kt of TNT (50 TJ) nuclear blast at 7.5 km. The nearest known magnetar to earth is 1E 2259+586, 4 kiloparsecs (13,000 light years) away.
SGR 1806-20 lies at the core of radio nebula G10.0-0.3 and is a component of cluster 1806-20, itself a component of W31, one of the largest H II regions in the Milky Way. Cluster 1806-20 is made up of some highly unusual stars, including at least two carbon-rich Wolf-Rayet stars
(WC9d and WCL), two blue hypergiants
, and one of the brightest/massive stars in the galaxy LBV 1806-20
- * As measured by various space-based and land-based astronomical observatories, including the Swift spacecraft.