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 angul ar momentum that centrifugal support against self-gravity becomes impossibl e, 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 fo rms sweeps the medium surrounding the remnant, and the quickly spinning rem nant loses energy through magnetic dipole radiation at a rate exceeding its Eddington luminosity by some 4 orders of magnitude. The implosion is adiab atic, because neutrinos have short mean free paths, and silent, given the p rompt 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 m echanism of energy extraction is via the conversion of the Poynting flux (d ue to the large-scale magnetic field locked into the minitorus) into a magn etized relativistic wind. Occasionally, this model will produce quickly dec aying or nondetectable afterglows.