We report our observations of very bright prompt optical and reverse shock ( RS ) optical emission of GRB 140512A and analyze its multi-wavelength data observed with the Swift and Fermi missions .
It is found that the joint 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 prompt gamma-rays can be fit by a single power-law with index -1.32 \pm 0.01 .
Our empirical fit to the afterglow lightcurves indicates that the observed bright optical afterglow with R=13.00 mag at the peak time is consistent with predictions of the RS and forward shock ( FS ) emission of external shock models .
Joint optical-X-ray afterglow spectrum is well fit with an absorbed single power-law , with an index evolving with time from -1.86 \pm 0.01 at the peak time to -1.57 \pm 0.01 at late epoch , which could be due to the evolution of the ratio of the RS to FS emission fluxes .
We fit the lightcurves with standard external models , and derive the physical properties of the outflow .
It is found that the ratio R _ { B } \equiv \epsilon _ { B,r } / \epsilon _ { B,f } is 8187 , indicating a high magnetization degree in the RS region .
Measuring the relative radiation efficiency with R _ { e } \equiv \epsilon _ { e,r } / \epsilon _ { e,f } , we have R _ { e } = 0.02 , implying the radiation efficiency of the RS is much lower than that in FS .
We also show that the R _ { B } of GRBs 990123 , 090102 , and 130427A are similar to that of GRB 140512A and their apparent difference may be mainly attributed to the difference of the jet kinetic energy , initial Lorentz factor , and medium density among them .