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A number of gamma-ray bursts (GRBs) exhibit the late simultaneous bumps intheir optical and Xray afterglows around the jet break. Its origin is unclear.Based on the following two facts, we suggest that this feature may sound atransition of circum-burst environment from a free-wind medium to a homogeneousmedium. (I) The late bump followed by a steep decay is strongly reminiscent ofthe afterglows of GRB 170817A, which is attributed to an off-axis observedexternal-forward shock (eFS) propagating in an interstellar medium. (II)Observations seem to feature a long shallow decay before the late optical bump,which is different from the afterglow of GRB 170817A. In this paper, we studythe emission of an eFS propagating in a free-to-shocked wind for on/off-axisobservers, where the mass density in the shocked-wind is almost constant. Thelate simultaneous bumps/plateaux in the optical and X-ray afterglows are reallyfound around the jet break for high-viewing-angle observers. Moreover, there isa long plateau or shallow decay before the late bump in the theoreticallight-curves, which is formed during the eFS propagating in the free-wind. Forlow-viewing-angle observers, the above bumps appear only in the situation thatthe structured jet has a low characteristic angle and the deceleration radiusof the on-axis jet flow is at around or beyond the free-wind boundary. Asexamples, the X-ray and optical afterglows of GRBs 120326A, 120404A, and100814A are fitted. We find that an off-axis observed eFS in a free-to-shockedwind can well explain the afterglows in these bursts.

Late Afterglow Bump/Plateau around the Jet Break: Signature of a free-to-shocked wind Environment in Gamma-ray Burst