(Katsuaki Asano on behalf of the Fermi/LAT collaboration)
The origin of the GeV emission in the early phase from gamma-ray bursts (GRBs) is still unclear, in spite of many observations and researches in the past. One of the major possible scenarios for the physical origin is the external-shock model, which can generally explain the behavior in the gamma-ray band such as delayed and temporally extended emission. High-energy emission from GRB 170405A was detected by Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) whose joint energy range coverage spans from 8 keV to 300 GeV. During the prompt emission phase, the GRB exhibits a delayed onset of the gamma-ray emission with respect to the X-ray emission by ~20 s, followed by a temporally extended emission lasting ~1000 s, which are common behaviors observed in other GRBs. In addition, ~200 s after the start of the GeV emission, a clear optical onset was detected by Swift/UVOT. If the optical onset was caused by the external shock, the GeV emission cannot be explained by the external-shock scenario in the context of the standard synchrotron shock model. Thus, our finding would shed new light on understanding of the gamma-ray emission mechanism of GRBs. We report the temporal and spectral observational results and discuss the physical origin of the gamma-ray emission.