Exploring Broadband GRB Behavior during γ-Ray Emission

The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the γ-ray emission of Swift events GRB 051109A and GRB 051111. Both data sets have continuous coverage at high signal-to-noise levels from the prompt phase onward, and thus the early observations are readily compared to the Swift XRT and BAT high-energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB 051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the γ-rays, using the γ-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components—one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB 051111—is not compatible with a simple "external-external" shock model for the GRB and its afterglow.