Soft -ray repeaters (SGRs) are 'magnetars', a small class of slowly spinning neutron stars with extreme surface magnetic fields, B 10¹⁵ gauss (refs 1, 2−3). On 27 December 2004, a giant flare⁴ 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 globe⁹. 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 source¹⁰. 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 10⁴³ ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.

1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
2. NASA/Marshall Space Flight Center; and
3. Universities Space Research Association, NSSTC, XD-12, 320 Sparkman Drive, Huntsville, Alabama 35805, USA
4. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, PO Box 20450, Stanford, California 94309, USA
5. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA
6. Department of Physics, Ben Gurion University, POB 653, Beer Sheva 84105, Israel
7. Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
8. Institute for Advanced Study, Einstein Drive, Princeton, New Jersey 08540, USA
9. School of Physics, University of Sydney, New South Wales 2006, Australia
10. University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
11. School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
12. Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
13. Australia Telescope National Facility, CSIRO, PO Box 76, Epping, New South Wales 1710, Australia
14. Los Alamos National Laboratory, PO Box 1663, Los Alamos, New Mexico 87545, USA
15. NASA/Goddard Space Flight Center, Code 661, Greenbelt, Maryland 20771, USA

Correspondence to: B. M. Gaensler¹ Correspondence and requests for materials should be addressed to B.M.G. (Email: bgaensler@cfa.harvard.edu).

Received 18 January 2005; Accepted 17 February 2005

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