ÃÛÌÒTV

We explore the analogy with the axisymmetric pulsar and derive the electromagnetic spindown of a rotating magnetized black hole after its formation in the core collapse of a supermassive star. The spindown shows two characteristic phases, an early Blandford-Znajek phase that lasts a few hundred seconds, and a late Kerr-Newman afterglow phase that lasts much longer. During the first phase, the spindown luminosity decreases almost exponentially, whereas during the afterglow phase it decreases as t^-4/3. The product of the exponential decay time times the maximum burst luminosity is equal to the total available black hole reducible rotational energy. Thus, a measurement of the former may allow us to use gamma-ray bursts as standard candles in cosmology.