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The origin of GeV emission from the early epoch of gamma-ray bursts (GRBs) isa widely discussed issue. The long gamma-ray burst GRB 170405A, observed by theFermi Gamma-ray Space Telescope showed high-energy emission delayed by $\sim$20s with respect to the X-ray emission, followed by temporally fading gamma-rayemission lasting for $\sim$1,000 s, as commonly observed in high-energy GRBs.In addition, a high-energy spectral cutoff at $\sim$50 MeV was detected in theprompt emission phase. If this feature is caused by pair-production opacity,the bulk Lorentz factor of the GRB ejecta can be estimated to be $\Gamma_{\rmbulk}$ = 170-420. Simultaneously with Fermi, GRB 170405A was observed bySwift/Burst Alert Telescope (BAT) and X-ray telescope (XRT), and a clearoptical onset was detected $\sim$200 s after the burst by Swift/UltraVioletOptical Telescope (UVOT). By coupling the deceleration time to the derived bulkLorentz factor, the deceleration time was found to correspond to the delayedonset in the optical band. While the delayed onset in the optical band isevidence that this emission had an external shock origin, the temporallyextended emission in the GeV band before the optical onset is hard to reconcilewith the standard synchrotron emission from the same external shock. This mayimply that the common feature of GeV emission with a power-law decay does notnecessarily have the same origin of the optical afterglow in all Fermi/LATGRBs, particularly in their early epoch. Another emission mechanism to explainthe GeV emission in GRB 170405A can be required such as an internal-shock orinverse Compton emission.

Physical origin of GeV emission in the early phase of GRB 170405A: Clues from emission onsets with multi-wavelength observations