1. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej, DK-2100 Copenhagen Ø, Denmark
2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
3. ESO Santiago, Casilla 19001, Santiago 19, Chile
4. Instituto de Astrofisica de Andalucía (IAA-CSIC), PO Box 3004, E-18080 Granada, Spain
5. Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
6. NASA/Marshall Space Flight Center, National Space Science Technology Center, XD-12, 320 Sparkman Drive, Huntsville, Alabama 35805, USA

Correspondence to: Jens Hjorth¹ Correspondence and requests for materials should be addressed to J.H. (Email: jens@astro.ku.dk).

It has long been known that there are two classes¹ of -ray bursts (GRBs), mainly distinguished by their durations. The breakthrough in our understanding of long-duration GRBs (those lasting more than 2 s), which ultimately linked them with energetic type Ic supernovae^{2, 3, 4}, came from the discovery of their long-lived X-ray⁵ and optical^{6, 7} 'afterglows', when precise and rapid localizations of the sources could finally be obtained. X-ray localizations have recently become available^{8, 9} for short (duration <2 s) GRBs, which have evaded optical detection for more than 30 years. Here we report the first discovery of transient optical emission (R-band magnitude 23) associated with a short burst: GRB 050709. The optical afterglow was localized with subarcsecond accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and X-ray¹⁰ afterglow properties 34 h after the GRB are reminiscent of the afterglows of long GRBs, which are attributable to synchrotron emission from ultrarelativistic ejecta. We did not, however, detect a supernova, as found in most nearby long GRB afterglows, which suggests a different origin for the short GRBs.

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