title: An expanding radio nebula produced by a giant flare from the magnetar SGR 1806-20.
abstract: Soft gamma-ray repeaters (SGRs) are 'magnetars', a small class of slowly spinning neutron stars with extreme surface magnetic fields, B approximately 10(15) gauss (refs 1 , 2 -3). On 27 December 2004, a giant flare was detected from the magnetar SGR 1806-20 (ref. 2), only the third such event recorded. This burst of energy was detected by a variety of instruments and even caused an ionospheric disturbance in the Earth's upper atmosphere that was recorded around the globe. 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. 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 x 10(43) ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. bgaensler@cfa.harvard.edu
authors.forename: B M
authors.initials: BM
authors.lastname: Gaensler
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authors.lastname: Kouveliotou
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authors.initials: JD
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authors.lastname: Taylor
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authors.lastname: Eichler
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authors.initials: RA
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authors.lastname: Granot
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authors.lastname: Ramirez-Ruiz
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authors.lastname: Hunstead
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authors.lastname: Campbell-Wilson
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authors.lastname: van der Horst
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authors.lastname: McLaughlin
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authors.lastname: Fender
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authors.lastname: Garrett
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authors.lastname: Newton-McGee
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authors.lastname: Palmer
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authors.lastname: Gehrels
authors.forename: P M
authors.initials: PM
authors.lastname: Woods