An expanding radio nebula produced by a giant flare from the magnetar SGR 1806–20

Flares back in fashion On 27 December last year, SGR1806–20, a soft γ-ray repeater in Sagittarius, released a giant flare that has been called the brightest explosion ever recorded. SGRs are X-ray stars that sporadically emit low-energy γ-ray bursts. They are thought to be magnetars: neutron stars with observable emissions powered by magnetic dissipation. Five papers in this issue report initial and follow-up observations of this event. The data are remarkable: for instance in a fifth of a second, the flare released as much energy as the Sun radiates in a quarter of a million years. Such power can be explained by catastrophic global crust failure and magnetic reconnection on a magnetar. Releasing a hundred times the energy of the only two previous SGR giant flares, this may have been a once-in-a-lifetime event for astronomers, and for the star itself. Soft γ-ray repeaters (SGRs) are ‘magnetars’, a small class of slowly spinning neutron stars with extreme surface magnetic fields, B ≈ 10 15 gauss (refs 1 , 2 –3 ). On 27 December 2004, a giant flare 4 was detected from the magnetar SGR 1806 - 20 (ref. 2 ), only the third such event recorded 5 , 6 . This burst of energy was detected by a variety of instruments 7 , 8 and even caused an ionospheric disturbance in the Earth's upper atmosphere that was recorded around the globe 9 . Here we report the detection of a fading radio afterglow produced by this outburst, with a luminosity 500 times larger than the only other detection of a similar source 10 . From day 6 to day 19 after the flare from SGR 1806 - 20, a resolved, linearly polarized, radio nebula was seen, expanding at approximately a quarter of the speed of light. To create this nebula, at least 4 × 10 43 ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.