GRB 110530A: peculiar broad bump and delayed plateau in early optical afterglows

We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow light curve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band light curve. Our joint spectral fits of the optical and X-ray data show that they are in the same regime, with a photon index of ∼1.70. The optical and X-ray afterglow light curves are well fitted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we find that the radiative efficiency of the gamma-ray burst jet is $\sim 1\%$ and the magnetization parameter of the afterglow jet is $<0.04$ with a derived extremely low ${ϵ}_B$ (the ratio of shock energy to the magnetic field) of $(1.64\pm 0.25)\times {10}^{-6}$. These results indicate that the jet may be matter dominated. A discussion on delayed energy injection from the accretion of the late fall-back material of its pre-supernova star is also presented.