SGR 1806-20 has a history of previous gamma bursts (SGR stands for Soft Gamma Repeater), and was known as a magnetar, a neutron star with extremely intense magnetic fields, of which there may be only a few dozen in the entire galaxy. Like all neutron stars, the star is greatly compacted, to about 20 km diameter, and rotates once every 7.5 seconds. Nevertheless, the intensity of the gamma-ray flare December 27th (the day after Earth shook itself with the largest earthquake in 40 years) took astronomers by surprise – it’s the most intense burst of radiation the galaxy has seen (from our perspective) since Kepler’s supernova of 1604, and 10,000 times brighter than this star had ever flared before.
Astronomers have been continuing to examine the star with radio-telescope observations since the December 27th burst. It looks like what happened was, in fact, a “star-quake”, which disrupted the magnetar’s magnetic field. This released the gamma rays and created a rapidly-expanding fireball of material from the star’s surface, which can be directly seen by these high-resolution telescopes. The fireball appears to be expanding at about 1/3 the speed of light. The energy release in the explosion seems to be a significant fraction of the energy stored in the stars intense magnetic fields.
Astronomical gamma rays can’t be detected through Earth’s atmosphere; it requires space-based instruments like the recently launched Swift GRB mission. This flash was so intense it was almost overwhelming for some instruments; a secondary flash reflected from the Moon was detected by instruments in Earth orbit, and even the Gamma Ray Spectrometer on the Mars Odyssey satellite detected the burst.
The relative proximity of the burst should yield a lot of interesting information about these rare events; on the other hand, if had been much closer, it could have caused even more death and destruction than the recent tsunami. The universe is a rather dangerous place…