The optical afterglow of the short γ-ray burst GRB 050709

Short gamma-ray bursts Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of Nature ), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole. It has long been known that there are two classes 1 of γ-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-ray 5 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