A. Black hole accretion
Several scenarios could lead to a black hole - massive accretion disk system. This could include mergers (NS-NS binaries [86, 276], NS-BH binaries , WD-BH binaries , BH-He-star binaries ) and models based on "failed supernovae" or "Collapsars" [242, 287, 443]. Narayan et al.  have recently shown that accretion theory suggests that from all the above scenarios only Collapsars could produce long bursts and only NS-NS (or NS-BH) mergers could produce short bursts. The basic idea is that the duration of the accretion depends on the size of the disk. So short burst must be produced by small disks and those are natureally produced in mergers. On the other hand long burst require large disks. However, those are inefficient. One can overcome this if we have a small disk that is fed continuously. In this case the efficiency can be large and the duration long. This happens naturally within the collapsar model.