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 (42,000 light-years) from Earth on the far side of the Milky Way in the constellation of Sagittarius. It has a diameter of no more than 20km (10miles) and rotates on its axis every 7.5 seconds (rotation speed at the equator on the surface)., SGR 1806-20 is the most highly magnetized object ever observed, with a magnetic field of over 1015 gauss (G) (1011 tesla) intensity[1] (compared to the Sun's 1–5 G and Earth's 0.25–0.65 G).
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 (GRB 041227).[2] 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 (1.0 J) than the Sun releases in 150,000 years (4 W × 4.8 s = 1.85 J). 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.[3]
A similar blast within 3 parsecs (10 light years) of Earth would severely affect the atmosphere, by destroying the ozone layer and causing mass extinction,[4] [5] 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.
SGR 1806−20 lies at the core of radio nebula G10.0-0.3 and is a member of an open cluster named after it, 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 LBV 1806-20, one of the brightest/most massive stars in the galaxy.
A 2024 paper posited that periodic gamma-ray and x-ray bursts from the pulsar are caused by a planet orbiting SGR 1806-20. This SGR 1806-20 b would be some kind of rocky world with a mass between 10-18 Earth masses with an orbital period of 398 days and a semi-major axis of 1.18 astronomical units. SGR 1806-20 b is notable for its eccentricity of 0.994, which would make it the most eccentric exoplanet known.[6] [7]