1. Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, California 91125, USA
2. Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA
4. University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
5. Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA
6. Racah Institute for Physics, The Hebrew University, Jerusalem 91904, Israel
7. Research School of Astronomy and Astrophysics, The Australian National University, Weston Creek, ACT 2611, Australia
8. Pomona College, 610 North College Avenue, Claremont, California 91711, USA
9. Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, Virginia 22903, USA
10. Astronomy Department, University of Texas, Austin, Texas 78731, USA
11. Gemini Observatory, 670 North A'ohoku Place, Hilo, Hawaii 97620, USA
12. Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
13. Department of Astronomy and Astrophysics, University of Toronto, Toronto, Ontario M5S 3H8, Canada
14. Department of Physics, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
15. Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A'ohoku Place, Hilo, Hawaii 96720, USA
16. Department of Astronomy, School of Science, Kyoto University, Sakyo-ku. Kyoto 606-8502, Japan
17. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA
18. Institute for Advanced Study, Princeton, New Jersey 08540, USA
19. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA

Correspondence to: D. B. Fox^1,2D. A. Frail³ Correspondence and requests for materials should be addressed to D.A.F. (Email: dfrail@nrao.edu) or D.B.F. (Email: dfox@astro.psu.edu).

Abstract

The final chapter in the long-standing mystery of the -ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

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