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Gamma-ray burst (GRB) 120729A was detected by Swift/BAT and Fermi/GBM, andthen rapidly observed by Swift/XRT, Swift/UVOT, and ground-based telescopes. Ithad a single long and smooth \gamma-ray emission pulse, which extendscontinuously to the X-rays. We report Lick/KAIT observations of the source, andmake temporal and spectral joint fits of the multiwavelength light curves ofGRB 120729A. It exhibits achromatic light-curve behavior, consistent with thepredictions of the external shock model. The light curves are decomposed intofour typical phases: onset bump (Phase I), normal decay (Phase II), shallowdecay (Phase III), and post-jet break (Phase IV). The spectral energydistribution (SED) evolves from prompt \gamma-ray emission to the afterglowwith photon index from $\Gamma_{\rm \gamma}=1.36$ to $\Gamma \approx 1.75$.There is no obvious evolution of the SED during the afterglow. Themultiwavelength light curves from \gamma-ray to optical can be well modeledwith an external shock by considering energy injection, and a time-dependentmicrophysics model with $\epsilon_B\propto t^{\alpha_B}$ for the emission atearly times, $T < T_{\rm 0} + 157$~s. Therefore, we conclude that both theprompt \gamma-ray emission and afterglow of GRB 120729A have the same externalshock physical origin. Our model indicates that the $\epsilon_B$ evolution canbe described as a broken power-law function with $\alpha_{\rm B,1} = 0.18 \pm0.04$ and $\alpha_{\rm B,2} = 0.84 \pm 0.04$. We also systematicallyinvestigate single-pulse GRBs in the Swift era, finding that only a smallfraction of GRBs (GRBs 120729A, 051111, and 070318) are likely to originatefrom an external shock for both the prompt \gamma-ray emission and afterglow.

GRB120729A: external shock origin for both the prompt gamma-ray emission and afterglow