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We report our observations of very bright prompt optical and reverse shock(RS) optical emission of GRB 140512A and analyze its multi-wavelength dataobserved with the {\em Swift} and {\em Fermi} missions. It is found that thejoint optical-X-ray-gamma-ray spectrum with our first optical detection(R=13.09 mag) at $T_0+136$ seconds during the second episode of the promptgamma-rays can be fit by a single power-law with index $-1.32\pm 0.01$. Ourempirical fit to the afterglow lightcurves indicates that the observed brightoptical afterglow with R=13.00 mag at the peak time is consistent withpredictions of the RS and forward shock (FS) emission of external shock models.Joint optical-X-ray afterglow spectrum is well fit with an absorbed singlepower-law, with an index evolving with time from $-1.86\pm 0.01$ at the peaktime to $-1.57\pm 0.01$ at late epoch, which could be due to the evolution ofthe ratio of the RS to FS emission fluxes. We fit the lightcurves with standardexternal models, and derive the physical properties of the outflow. It is foundthat the ratio $R_B\equiv\epsilon_{\rm B, r}/\epsilon_{\rm B, f}$ is 8187,indicating a high magnetization degree in the RS region. Measuring the relativeradiation efficiency with $R_e\equiv\epsilon_{\rm e, r}/\epsilon_{\rm e, f}$,we have $R_e= 0.02$, implying the radiation efficiency of the RS is much lowerthan that in FS. We also show that the $R_B$ of GRBs 990123, 090102, and130427A are similar to that of GRB 140512A and their apparent difference may bemainly attributed to the difference of the jet kinetic energy, initial Lorentzfactor, and medium density among them.

Very Bright Prompt and Reverse Shock Emission of GRB 140512A