TESTS AND CONSEQUENCES OF DISK PLUS HALO MODELS OF GAMMA-RAY BURST SOURCES

The gamma-ray burst observations made by the Burst and Transient Sourc e Experiment (BATSE) and by previous experiments are still consistent with a combined Galactic disk (or Galactic spiral arm) plus extended G alactic halo model. Testable predictions and consequences of the disk plus halo model are discussed here; tests performed on the expanded BA TSE database in the future will constrain the allowed model parameters and may eventually rule out the disk plus halo model. Using examples, it is shown that if the halo has an appropriate edge, BATSE will neve r detect an anisotropic signal from the halo of the Andromeda galaxy. A prediction of the disk plus halo model is that the fraction of the b ursts observed to be in the ''disk'' population rises as the detector sensitivity improves. A careful reexamination of the numbers of bursts in the two populations for the pre-BATSE databases could rule out thi s class of models. Similarly, it is predicted that different satellite s will observe different relative numbers of bursts in the two classes for any model in which there are two different spatial distributions of the sources, or for models in which there is one spatial distributi on of the sources that is sampled to different depths for the two clas ses. An important consequence of the disk plus halo model is that for the birthrate of the halo sources to be small compared to the birthrat e of the disk sources, it is necessary for the halo sources to release many orders of magnitude more energy over their bursting lifetime tha n the disk sources. The halo bursts must also be much more luminous th an the disk bursts; if this disk-halo model is correct, it is necessar y to explain why the disk sources do not produce halotype bursts.