2006A&A...454L.119J

We present a discovery and observation of an extraordinarily bright prompt optical emission of the GRB060117 obtained by a wide-field camera atop the robotic telescope FRAM of the Pierre Auger Observatory from 2 to 10 min after the GRB. We found rapid average temporal flux decay of α=-1.7±0.1 and a peak brightness R=10.1mag. Later observations by other instruments set a strong limit on the optical and radio transient fluxes, unveiling an unexpectedly rapid further decay. We present an interpretation featuring a relatively steep electron-distribution parameter p≃3.0 and providing a straightforward solution for the overall fast decay of this optical transient as a transition between reverse and forward shock.