SGR 1806−20
SGR 1806−20 is a magnetar, a type of neutron star with a very powerful magnetic field, that was discovered in 1979 and identified as a soft gamma repeater. SGR 1806−20 is located about 13 kiloparsecs from Earth on the far side of the Milky Way in the constellation of Sagittarius. It has a diameter of no more than and rotates on its axis every 7.5 seconds., SGR 1806-20 is the most highly magnetized object ever observed, with a magnetic field of over 1015 gauss intensity.
Explosion
Forty-two thousand years after a starquake occurred on the surface of SGR 1806-20, the radiation from the resultant explosion reached Earth on December 27, 2004. In terms of gamma rays, the burst had an absolute magnitude of around −29. It was the brightest event known to have been sighted on this planet from an origin outside the Solar System until GRB 080319B. The magnetar released more energy in one-tenth of a second than the Sun releases in 150,000 years. Such a burst is thought to be the largest explosion observed in this galaxy by humans since the SN 1604, a supernova observed by Johannes Kepler in 1604. The gamma rays struck Earth's ionosphere and created more ionization, which briefly expanded the ionosphere. The quake was equivalent to a magnitude 32 on the Richter scale.A similar blast within 3 parsecs of Earth would severely affect the atmosphere, by destroying the ozone layer and causing mass extinction, and be similar in effect to a 12-kiloton nuclear blast at. The nearest known magnetar to Earth is 1E 1048.1-5937, located 9,000 light-years away in the constellation Carina.