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Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806–20


It was established over a decade ago that the remarkable high-energy transients known as soft γ-ray repeaters (SGRs) are located in our Galaxy1,2 and originate from neutron stars with intense (≤ 1015G) magnetic fields—so-called ‘magnetars’3. On 27 December 2004, a giant flare4 with a fluence5 exceeding 0.3 erg cm-2 was detected from SGR 1806–20. Here we report the detection of a fading radio counterpart to this event. We began a monitoring programme from 0.2 to 250 GHz and obtained a high-resolution 21-cm radio spectrum that traces the intervening interstellar neutral hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806 - 20: the source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If correct, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for γ-ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.

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Figure 1: Broadband temporal behaviour of the transient radio source coincident with SGR 1806–20.
Figure 2: Cold atomic and molecular hydrogen spectra towards SGR 1806 - 20.


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ATCA is funded by the Commonwealth of Australia for operations as a National Facility managed by CSIRO. We thank K. Newton-McGee and B. Gaensler for scheduling and performing observations with the ATCA. GMRT is run by the National Centre for Radio Astrophysics—Tata Institute of Fundamental Research, India. We thank the GMRT staff and in particular C. H. Ishwara-Chandra and D. V. Lal for help with observations and analysis. The VLA is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. NMA is a branch of the National Astronomical Observatory, National Institutes of Natural Sciences, Japan. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We thank A. Weiss from IRAM for help with the observations. We gratefully acknowledge discussions with S. Corbel, S. S. Eikenberry and R. Sari. Our work is supported in part by the NSF and NASA.

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Correspondence to P. B. Cameron.

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Cameron, P., Chandra, P., Ray, A. et al. Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806–20. Nature 434, 1112–1115 (2005).

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