Giant flare illuminates the Earth

6 June 2005

On 27 December 2004, one day after the devastating tsunami in the Indian Ocean, the Earth was illuminated by the biggest splash of light ever recorded from outside the solar system. For 0.2 s, the flare released as much energy as has been radiated by the Sun in 250,000 years. Five papers recently published in Nature describe this event.

The source of this giant flare was identified as the soft gamma repeater SGR 1806-20 located at some 50,000 light-years on the opposite side of the galaxy. Soft gamma repeaters flare up randomly and release gamma rays with a slightly softer spectrum than usual gamma-ray bursts. Only four such objects are known and a giant flare has now been detected for three of them.

The 2004 event is, however, more than an order of magnitude brighter than those recorded previously, on 5 March 1979 (SGR 0525-66) and 27 August 1998 (SGR 1900+14). Soft gamma repeaters are thought to be “magnetars” – isolated neutron stars with an extreme magnetic field that reaches 100 billion T at the surface of the star.

The most likely interpretation of this dramatic outburst is a magnetic reconnection, similar to – but much more powerful than – solar flares. Its unusual strength may be related to a quake in the crust at the surface of the neutron star. According to Kevin Hurley and collaborators, the opening of the magnetic field lines outward launched a hot fireball, a thermal pair plasma emitting the quasi-blackbody spectrum observed during the initial gamma-ray spike with a kT value of around 200 keV (Hurley et al.).

This prompt emission, first reported by ESA’s INTEGRAL satellite, was followed by an exponential decay lasting about 400 s. On top of the general trend, very clear oscillations have been recorded with a period of 7.56 s, the previously known spin period of the magnetar SGR 1806-20.

On 3 January 2005, the Very Large Array (VLA) in New Mexico detected a radio source at the position of the giant flare (B M Gaensler et al.). Further observations over the following weeks showed that the radio-emitting fireball was expanding at roughly one-third the speed of light. The source is not spherical as suggested by polarization measures. Indeed, it seems to be elongated, with its shape changing from one observation to the other.

The extraordinary luminosity of the flare of December 2004 suggests that similar events could have been seen in nearby galaxies. Such an event would look like one of the many short gamma-ray bursts (GRBs) detected by the Burst And Transient Source Experiment (BATSE) in the 1990s. Hurley and collaborators therefore speculate that about 40% of the short GRBs detected by BATSE could be due to such giant flares from magnetars. However, the suggestion by P Cameron and colleagues that SGR 1806-20 is at only about half the distance assumed for this estimate makes it less likely that such events could explain a significant fraction of the still mysterious GRBs (Cameron et al.).

Further reading

P B Cameron et al. 2005 Nature 434 1112.
B M Gaensler et al. 2005 Nature 434 1104.
K Hurley et al. 2005 Nature 434 1098.
D M Palmer et al. 2005 Nature 434 1107.
T Terasawa et al. 2005 Nature 434 1110.

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