Gamma-ray burst models

I consider various possibilities for making gamma–ray bursts, particularly from close binaries. In addition to the much–studied neutron star + neutron star and black hole + neutron star cases usually considered good candidates for short–duration bursts, there are other possibilities. In particular neutron star + massive white dwarf has several desirable features. These systems are likely to produce long–duration GRBs, in some cases definitely without an accompanying supernova, as observed recently. This class of burst would have a strong correlation with star formation, and occur close to the host galaxy. However rare members of the class need not be near star–forming regions, and could have any type of host galaxy. Thus a long–duration burst far from any star–forming region would also be a signature of this class. Estimates based on the existence of a known progenitor suggest that this type of GRB may be quite common, in agreement with the fact that the absence of a supernova can only be established in nearby bursts.