<P> Although some light curves can be roughly reproduced using certain simplified models, little progress has been made in understanding the full diversity observed . Many classification schemes have been proposed, but these are often based solely on differences in the appearance of light curves and may not always reflect a true physical difference in the progenitors of the explosions . However, plots of the distribution of the observed duration for a large number of gamma - ray bursts show a clear bimodality, suggesting the existence of two separate populations: a "short" population with an average duration of about 0.3 seconds and a "long" population with an average duration of about 30 seconds . Both distributions are very broad with a significant overlap region in which the identity of a given event is not clear from duration alone . Additional classes beyond this two - tiered system have been proposed on both observational and theoretical grounds . </P> <P> Events with a duration of less than about two seconds are classified as short gamma - ray bursts . These account for about 30% of gamma - ray bursts, but until 2005, no afterglow had been successfully detected from any short event and little was known about their origins . Since then, several dozen short gamma - ray burst afterglows have been detected and localized, several of which are associated with regions of little or no star formation, such as large elliptical galaxies and the central regions of large galaxy clusters . This rules out a link to massive stars, confirming that short events are physically distinct from long events . In addition, there has been no association with supernovae . </P> <P> 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 miles--four times the Earth's diameter). This further suggests a very compact object as the source . 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 . </P> <P> The origin of short GRBs in kilonovae was confirmed when short GRB 170817A was detected only 1.7 s after the detection of gravitational wave GW170817, which was a signal from the merger of two neutron stars . </P>

Short duration gamma-ray bursts are thought to be caused by