  Gamma-ray bursts (GRBs) are believed to be powered by the electromagnetic
extraction of spin energy from a black hole endowed with a magnetic field
supported by electric currents in a surrounding disk (Blandford & Znajek 1977).
A generic feature of this mechanism is that, under certain fairly general
assumptions, the energy loss rate decays exponentially. In this work, we are
looking precisely for such exponential decay in the lightcurves of long
duration GRBs observed with the XRT instrument on the Swift satellite. We found
out that almost 30 % of XRT lightcurves show such behavior before they reach
the afterglow plateau. According to Blandford & Znajek, the duration of the
burst depends on the magnetic flux accumulated on the event horizon. This
allows us to estimate the surface magnetic field of a possible progenitor. Our
estimations are consistent with magnetic fields observed in Wolf-Rayet stars.
