The true nature of these objects was initially unknown, and the leading hypothesis was that they originated from the mergers of binary neutron stars or a neutron star with a black hole. Such mergers were theorized to produce kilonovae, and evidence for a kilonova associated with GRB 130603B was seen. The mean duration of these events of 0.2 seconds suggests (because of causality) a source of very small physical diameter in stellar terms; less than 0.2 light-seconds (about 60,000 km or 37,000 milesfour times the Earth's diameter). The observation of minutes to hours of X-ray flashes after a short gamma-ray burst is consistent with small particles of a primary object like a neutron star initially swallowed by a black hole in less than two seconds, followed by some hours of lesser energy events, as remaining fragments of tidally disrupted neutron star material (no longer neutronium) remain in orbit to spiral into the black hole, over a longer period of time. A small fraction of short gamma-ray bursts are probably produced by giant flares from soft gamma repeaters in nearby galaxies.