General Astronomy/Long-Soft and Short-Hard Classifications

Long gamma-ray bursts


Most observed events have a duration of greater than two seconds and are classified as long gamma-ray bursts. Because these events constitute the majority of the population and because they tend to have the brightest afterglows, they have been studied in much greater detail than their short counterparts. Almost every well-studied long gamma-ray burst has been linked to a galaxy with rapid star formation, and in many cases to a core-collapse supernova as well. Long GRB afterglow observations, at high redshift, are also consistent with the GRB having originated in star-forming regions. A unique gamma ray emission event, GRB 110328A, lasting more than two and a half months was observed starting March 28, 2011, originating from the center of a small galaxy at redshift z = 0.3534. The event is interpreted as a supermassive black hole devouring a star, most likely a white Dwarf and emitting its beam of radiation towards Earth. It could thus be viewed as a temporarily active blazar (a type of Quasar).

Short gamma-ray bursts


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. The true nature of these objects (or even whether the current classification scheme is accurate) remains unknown, although the leading hypothesis is that they originate from the mergers of binary neutron stars or a neutron star with a black hole. 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.