Gamma-Ray Bursts from Neutron Star Phase Transitions

The phase-transition induced collapse of a neutron star to a more compact configuration (typically a "strange" star) and the subsequent core bounce are often invoked as a model for gamma-ray bursts. We present the results of numerical simulations of this kind of event using realistic neutrino physics and a high-density equation of state. The nature of the collapse itself is represented by the arbitrary motion of a piston deep within the star, but if any shock is to develop, the transition, or at least its final stages, must occur in less than a sonic time. Fine surface zoning is employed to adequately represent the acceleration of the shock to relativistic speeds and to determine the amount and energy of the ejecta. We find that these explosions are far too baryon-rich (Mejecta ~ 0.01 M☉) and have much too low an energy to explain gamma-ray bursts. The total energy of the ejecta having relativistic Γ ≳ 40 is less than 1046 ergs, even in our most optimistic models (deep bounce, no neutrino losses or photodisintegration). However, the total energy of all the ejecta, mostly mildly relativistic, is ~1051 ergs, and, if they occur, these events might be observed. They would also contribute to Galactic nucleosynthesis, especially the r-process, even though the most energetic layers are composed of helium and nucleons, not heavy elements.