Abstract Observations imply that long γ-ray bursts (GRBs) originate from the explosions of massive stars, therefore they may occur in the molecular clouds where their progenitors were born. We show that the prompt optical-UV emission from GRBs may be delayed due to dust extinction, which can explain the observed optical delayed onset and fast rise in GRB 080319B well. The density and the size of the molecular cloud around GRB 080319B are roughly constrained to be ~ 103 cm−3 and ~ 8 pc, respectively. We also investigate other GRBs with prompt optical-UV data, and find similar values of the densities and sizes of the local molecular clouds. Future observations of prompt optical-UV emission from GRBs on a timescale of subseconds, e.g. by UFFO-Pathfinder and SVOM-GWAC, will provide more evidence and probes of the local environments of GRBs.

Keywords radiation mechanisms: non-thermal — gamma-rays: bursts — dust: ex- tinction

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