Trainsient Accretion Disk and Energy Mechanism of Gamma Ray Bursts

We suggest that a rotating massive black hole (l0⁶ M _O) located at an inactive galaxy may convert its host into a transient active phase by capturing and disrupting a star. During this period, a transient accretion disk is formed and a strong transient magnetic field can be produced in the inner boundary of the accretion disk. A large amount of rotational energy of the black hole is extracted and released in the ultra relativistic jet with a bulk Lorentz factor larger than l0³ via Blandford-Znajek process. The relativistic jet energy can be converted into γ-ray radiation in the shock region located at a distance of about 1.4 x 10¹⁶ cm via the external shock mechanism. The observed properties of GRB971214 is used to illustrate our model.