Evolution of a black hole at the center of a Gamma-ray burst

Using a simplified model of a black-hole accretion-disk system which is dominated by the Poynting flux, we discuss the evolution of the central black-hole which is supposed to be powering Gamma-ray burst (GRB). It is demonstrated explicitly that there is a lower limit on the angular momentum parameter for a given GRB energy, It is found that the most energetic GRBs call only accomodate relatively rapidly rotating black-holes at the center. For a set of GRBs for which the isotropic energies and T(90)s are known, the effects of file disk mass and the magnetic field on the horizon are dicussed quantitatively. It is found that the magnetic Field has little influence on the energy, but affects the GRB duration, as expected. The role of the disk mass is found to be significant in determining both the energy and the duration.