1. Division of Physics, Mathematics and Astronomy, 105-24; and
2. Theoretical Astrophysics, 130-33, California Institute of Technology, Pasadena, California 91125, USA
3. Tata Institute of Fundamental Research, Mumbai 400 005, India
4. Joint Astronomy Programme, Indian Institute of Science, Bangalore 560 012, India
5. National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA
6. Australia Telescope National Facility, CSIRO, PO Box 76, Epping, New South Wales 1710, Australia
7. Shanghai Astronomical Observatory 80, Nandan Road, Shanghai 200030, China
8. Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Minamisaku, Nagano 384-1305, Japan
9. Department of Physics, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
10.  Max Planck Institut für Radioastronomie, Auf dem Hügel 69; and
11.  University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Correspondence to: P. B. Cameron¹ Correspondence and requests for materials should be addressed to P.B.C. (Email: pbc@astro.caltech.edu).

Abstract

It was established over a decade ago that the remarkable high-energy transients known as soft -ray repeaters (SGRs) are located in our Galaxy^{1, 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 -ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.

1. Division of Physics, Mathematics and Astronomy, 105-24; and
2. Theoretical Astrophysics, 130-33, California Institute of Technology, Pasadena, California 91125, USA
3. Tata Institute of Fundamental Research, Mumbai 400 005, India
4. Joint Astronomy Programme, Indian Institute of Science, Bangalore 560 012, India
5. National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA
6. Australia Telescope National Facility, CSIRO, PO Box 76, Epping, New South Wales 1710, Australia
7. Shanghai Astronomical Observatory 80, Nandan Road, Shanghai 200030, China
8. Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Minamisaku, Nagano 384-1305, Japan
9. Department of Physics, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
10.  Max Planck Institut für Radioastronomie, Auf dem Hügel 69; and
11.  University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Correspondence to: P. B. Cameron¹ Correspondence and requests for materials should be addressed to P.B.C. (Email: pbc@astro.caltech.edu).

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