A neutron star is a type of compact star that is formed from the gravitational collapse of the core of a massive star during a supernova explosion. A typical neutron star has a diameter of around 20 kilometres and a mass that generally exceeds the mass of our Sun. In comparison, our Sun has a diameter of 1.392 million kilometres. This incredibly compact configuration for a neutron star means that just a single cubic centimetre of its material packs a mass of around a billion metric tons! The extreme compactness of a typical neutron star also gives it a surface gravity that is over 100 billion times the surface gravity on the Earth and an escape velocity of around one third the speed of light!
A magnetar is an exceedingly rare type of neutron star which possesses an extremely powerfully magnetic field. In fact, the magnetic fields of magnetars are the strongest known in the universe as these magnetic fields have intensities on the order of between a billion to a trillion teslas. For comparison, the strength of the Earth’s magnetic field is about 30 microteslas while the strongest permanent magnets can generate magnetic fields of up 5 teslas. Magnetars are so rare that less than 15 of them are known. These exotic stars give rise to occasional burst of X-rays and gamma-rays, thus manifesting themselves as either Soft Gamma-ray Repeaters (SGRs) or Anomalous X-ray Pulsars (AXPs). SGRs are generally more energetic than AXPs and the bursting/flaring events from magnetars can be roughly classified into 3 types – short bursts, intermediate flares and giant flares. Giant flares are far more energetic than the short bursts and intermediate flares, and only 3 giant flares have been recorded in the decades of monitoring high energy astrophysical events since the 1970s.
SGR 1806-20 is a magnetar that is located around 50 thousand light years away, on the other side of the of the Milky Way galaxy. At this distance, it takes light 50 thousand years to travel from SGR 1806-20 to the Earth. The stellar neighbourhood of SGR 1806-20 contains some highly unusual stars, including one of the most massive and luminous star known in the Milky Way galaxy. What makes SGR 1806-20 unique is that this magnetar has the strongest magnetic field ever discovered for any object in the universe and this magnetar is also the progenitor for one of the 3 giant flares recorded so far. The strength of the magnetic field of SGR 1806-20 is estimated to be on the order of a whopping one trillion teslas!
On Monday 27 December 2004, an extremely energetic giant flare was detected from SGR 1806-20. This giant flare was so energetic that it saturated all but the least sensitive particle detectors regardless of where the detectors were pointed and this event became the brightest blast of gamma-rays ever detected from an astrophysical source. The giant flare from SGR 1806-20 is estimated to have released more than 2000 trillion trillion trillion joules of energy in the form of X-rays and gamma-rays. Almost all of the energy released from the giant flare was concentrated in an initial hard spike that lasted for around 0.2 seconds. This initial hard spike was then followed by a gradually decaying pulsating tail which shows about 50 cycles of high-amplitude pulsations over the duration of around 600 seconds. The high-amplitude pulsations show a period of 7.5 seconds and this period matches the rotational period of SGR 1806-20.
To place the amount of energy generated by the giant flare from SGR 1806-20 into perspective, the amount of power produced during the initial hard spike which lasted for around 0.2 seconds is on the order of a thousand times the combined luminosity of all the hundred of billions of stars in the Milky Way galaxy! In fact, the amount of energy produced during the 0.2 seconds of the initial hard spike is greater than the total amount of energy generated by our Sun over a period of 100 thousand years! Already, the amount of energy produced by our Sun in a single second is almost a million times the total worldwide energy consumption in 2009! The giant flare from SGR 1806-20 was so bright that even its echo off our Moon was detectable. Interestingly, if all the energy were converted into visible light, the giant flare would have been brighter than the full Moon during the 0.2 seconds duration of the initial hard spike! If the giant flare from SGR 1806-20 had occurred at a distance of 10 light years from the Earth, it will be similar to standing at a distance of 7.5 kilometres from a 15 kiloton nuclear explosion.
The giant flare detected on 27 December 2004 from SGR 1806-20 is hundreds of times more energetic than the two other known giant flare events. The release of such an immense amount of energy within such a short period of time managed to eject a significant amount of matter from the magnetar. The highly energetic ejecta formed an outflow which interacted with the external interstellar medium and produced a radio afterglow this is at least 500 times more luminous than the only other radio afterglow detected from a giant flare. Finally, it may be possible for ultra-high energy cosmic rays from the giant flare to be detected years after the event form the direction of SGR 1806-20, provided that the deflection of the ultra-high energy cosmic rays by galactic magnetic fields is not too large.