The Signature of the Blandford-Znajek Mechanism in GRB Light Curves.

In 1977, Blandford and Znajek showed how the spin energy of a rotating black hole may be extracted electromagnetically through a magnetic field that threads the black hole horizon. A characteristic feature of this mechanism is that, under certain fairly general conditions, the energy loss rate decays exponentially. We looked precisely for such behavior in the X-ray light curves of Long and Ultra Long duration Gamma-Ray Bursts (GRBs) observed with the XRT instrument on board the Swift satellite, and found that almost 30% of XRT light curves show an exponential decay before they reach the afterglow plateau. A similar behavior (Fast Rise Exponential Decay-FRED) was observed in 7-rays with the BATSE instrument aboard the CGRO satellite. We consider both of these findings as the signature of the Blandford-Znajek mechanism in action in the central engine of GRBs. [ABSTRACT FROM AUTHOR]