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The successful operation of dedicated detectors has brought us valuableinformation for understanding the central engine and the progenitor ofgamma-ray bursts (GRBs). For instance, the giant X-ray and optical bumps foundin some long-duration GRBs (e.g. GRBs 121027A and 111209A) imply that someextended central engine activities, such as the late X-ray flares, are likelydue to the fall-back of progenitor envelope materials. Here we systemicallysearch for long GRBs that consist of a giant X-ray or optical bump from theSwift GRB sample, and eventually we find 19 new possible candidates. Thefall-back accretion model could well interpret the X-ray and optical bump forall candidates within a reasonable parameter space. Six candidates showingsimultaneous bump signatures in both X-ray and optical observations, whichcould be well fitted at the same time when scaling down the X-ray flux intooptical by one order of magnitude, are consistent with the standard$F_{\nu}\propto\nu^{1/3}$ synchrotron spectrum. The typical fall-back radius isdistributed around $10^{10}\rm-10^{12}$ cm, which is consistent with thetypical radius of a Wolf-Rayet star. The peak fall-back accretion rate is inthe range of $\sim 10^{-11}-10^{-4}M_{\odot} \ \text{s} ^{-1}$ at time$\sim10^{2}- 10^{5}~\rm s$, which is relatively easy to fulfill as long as theprogenitor's metallicity is not too high. Combined with the sample we found,future studies of the mass supply rate for the progenitors with different mass,metallicity, and angular momentum distribution would help us to betterconstrain the progenitor properties of long GRBs.

Giant X-ray and optical Bump in GRBs: evidence for fall-back accretion model