  According to Blandford & Znajek (1977), energy can be extracted
electromagnetically from a rotating black hole, should the latter be endowed
with a magnetic field supported by electric currents in a surrounding disk. We
show that exact models of black hole magnetospheres produce Poynting flux that
decreases almost exponentially with time. We went through the Swift BAT-XRT
lightcurves and identified a subclass of GRBs that exhibits a clear exponential
decay over more than three orders of magnitude in flux (EDOHS GRBs). We
estimate the energy given-off in the X-rays and discuss a possible correlation
between the peak brightness of the X-ray prompt emission and its decay time. We
investigate a possible application of this result in high redshift Cosmology.
