Short-duration gamma-ray bursts (SGRBs) are intense flashes of cosmicgamma-rays, lasting less than ~2 s, whose origin is one of the great unsolvedquestions of astrophysics today. While the favoured hypothesis for theirproduction, a relativistic jet created by the merger of two compact stellarobjects (specifically, two neutron stars, NS-NS, or a neutron star and a blackhole, NS-BH), is supported by indirect evidence such as their host galaxyproperties, unambiguous confirmation of the model is still lacking. Mergers ofthis kind are also expected to create significant quantities of neutron-richradioactive species, whose decay should result in a faint transient in the daysfollowing the burst, a so-called "kilonova". Indeed, it is speculated that thismechanism may be the predominant source of stable r-process elements in theUniverse. Recent calculations suggest much of the kilonova energy should appearin the near-infrared (nIR) due to the high optical opacity created by theseheavy r-process elements. Here we report strong evidence for such an eventaccompanying SGRB 130603B. If this simplest interpretation of the data iscorrect, it provides (i) support for the compact object merger hypothesis ofSGRBs, (ii) confirmation that such mergers are likely sites of significantr-process production and (iii) quite possibly an alternative, un-beamedelectromagnetic signature of the most promising sources for direct detection ofgravitational waves.
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