AAS Meeting #193 - Austin, Texas, January 1999
Session 87. Gamma-Ray Bursts II
Oral, Friday, January 8, 1999, 2:00-3:30pm, Ballroom A

## [87.06] The Rates of Black-Hole Accretion Disk GRBs

C.L. Fryer (Lick Observatory, UC Santa Cruz)

The growing evidence that gamma-ray bursts (GRB) are cosmological in origin places strong requirements upon GRB mechanisms. A group of mechanisms is becoming the "standard" class of GRB models: rapidly accreting (\dot{M}\rm acc > 0.01 M\odot \, {\rm s-1}) accretion disks around stellar mass black holes. The geometry of the accretion disk facilitates efficient energy conversion (either by neutrino annihilation or magnetic fields) and low baryonic contamination (allowing the formation of highly relativistic jets). High accretion rates are required to obtain the energies (> 1051 {\rm ergs}) seen in gamma-ray bursts. Currently, 5 progenitors which produce rapidly accreting black holes have been proposed: merging double neutron star binaries, merging black hole + neutron star binaries, merging black hole + white dwarf binaries, collapsing rotating massive stars, and black holes inspiralling into helium companions.

All of these progenitor scenarios have passed the basic GRB energetics requirements. In these scenarios, beaming of roughly a factor of 10 is likely to occur. Although beaming lowers the demands of the energetics, it increases the required rate of any scenario. We present results of Monte-Carlo population synthesis studies of the 5 black-hole accretion disk GRB models, both to determine if the rates (with the effects of beaming) are sufficient to explain GRBs, but also to compare the relative rates to determine which scenario dominates. These simulations not only give the rates of the GRB progenitors, but also tell us when and where they produce GRB outbursts. As GRB statistics improve, these predictions can be used to rule out models.