The early optical emission of gamma-ray bursts gives an opportunity to understand the central engine and first stages of these events .
About 30 % of GRBs present flares whose origin is still a subject of discussion .
We present optical photometry of GRB 180620A with the COATLI telescope and RATIR instrument .
COATLI started to observe from the end of prompt emission at T + 39.3 s and RATIR from T + 121.4 s. We supplement the optical data with the X-ray light curve from Swift /XRT .
We observe an optical flare from T + 110 to T + 550 s , with a temporal index decay \alpha _ { \mathrm { O,decay } } = 1.32 \pm 0.01 , and a \Delta t / t = 1.63 , which we interpret as the signature of a reverse shock component .
After the initial normal decay the light curves show a long plateau from T + 500 to T + 7800 s both in X-rays and the optical before decaying again after an achromatic jet break at T + 7800 s. Fluctuations are seen during the plateau phase in the optical .
Adding to the complexity of GRB afterglows , the plateau phase ( typically associated with the coasting phase of the jet ) is seen in this object after the “ normal ” decay phase ( emitted during the deceleration phase of the jet ) and the jet break phase occurs directly after the plateau .
We suggest that this sequence of events can be explained by a rapid deceleration of the jet with t _ { d } \lesssim 40 s due to the high density of the environment ( \approx 100 cm ^ { -3 } ) followed by reactivation of the central engine which causes the flare and powers the plateau phase .