A gamma-ray burst model with small baryon contamination

We present a scenario ("supranova") for the formation of gamma-ray bursts (GRBs) occurring when a supramassive neutron star (SMNS) loses so much angular momentum that centrifugal support against self-gravity becomes impossible, and the star implodes to a black hole. This may be the most baryon-clean environment proposed so far, because the SN explosion in which the SMNS forms sweeps the medium surrounding the remnant, and the quickly spinning remnant loses energy through magnetic dipole radiation at a rate exceeding its Eddington luminosity by some 4 orders of magnitude. The implosion is adiabatic, because neutrinos have short mean free paths, and silent, given the prompt collapse of the polar caps. However, a mass of M-l approximate to 0.1 M-. in the equatorial belt can easily reach centrifugal equilibrium. The mechanism of energy extraction is via the conversion of the Poynting flux (due to the large-scale magnetic field locked into the minitorus) into a magnetized relativistic wind. Occasionally, this model will produce quickly decaying or nondetectable afterglows.