The bright optical flash from GRB 060117

Abstract

We present a discovery and observation of an extraordinarily bright prompt optical emission of the GRB 060117 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 and a peak brightness mag. 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 and providing a straightforward solution for the overall fast decay of this optical transient as a transition between reverse and forward shock.