Supermassive Stars as Gamma-Ray Bursters

We propose that the gravitational collapse of supermassive stars (M ∼> 5× 10 M⊙) could be a cosmological source of γ-ray bursts. Supermassive stars could form as an intermediate step in the collapse of relativistic star clusters, or in supermassive clouds with inefficient cooling. Supermassive stars would be de-stabilized by the Feynman-Chandrasekhar instability and could collapse to black holes, releasing a fraction of their huge gravitational binding energy as thermal neutrino pairs. (Their relic black holes could provide the seeds of the supermassive black holes observed at the center of many galaxies.) We show that the accompanying neutrino/antineutrino annihilation-induced heating could drive electron/positron “fireball” formation, relativistic expansion, and associated γ-ray emission. There are two major advantages of this model. (1) Supermassive star collapses are far more energetic than solar mass-scale catastrophic events such as neutron-star/neutron-star mergers; therefore, the conversion of gravitational energy to fireball kinetic energy in the supermassive star scenario need not be highly efficient, nor is it necessary to invoke directional beaming. Further, the cooling time of “afterglows” in our model is naturally long enough to accommodate observational constraints. (2) There is no need for actively star-forming galaxy hosts, since the formation/collapse of supermassive stars may not be tied to formation of ordinary stars. We explore other distinctive features of the supermassive star collapse model. Subject headings: gamma rays: bursts cosmology: observations and theory