GRB 120729A: External Shock Origin for Both the Prompt Gamma-Ray Emission and Afterglow

Abstract

Gamma-ray burst (GRB) 120729A was detected by Swift /BAT and Fermi /GBM, and then rapidly observed by Swift /XRT, Swift /UVOT, and ground-based telescopes. It had a single long and smooth γ -ray emission pulse, which extends continuously to the X-rays. We report Lick/KAIT observations of the source, and make temporal and spectral joint fits of the multiwavelength light curves of GRB 120729A. It exhibits achromatic light-curve behavior, consistent with the predictions of the external shock model. The light curves are decomposed into four typical phases: onset bump (Phase I), normal decay (Phase II), shallow decay (Phase III), and post-jet break (Phase IV). The spectral energy distribution (SED) evolves from prompt γ -ray emission to the afterglow with a photon index from Γ γ  = 1.36 to Γ ≈ 1.75. There is no obvious evolution of the SED during the afterglow. The multiwavelength light curves from γ -ray to optical can be well modeled with an external shock by considering energy injection, and a time-dependent microphysics model with ϵ B ∝ t α B for the emission at early times, T < T 0 + 157 s . Therefore, we conclude that both the prompt γ -ray emission and afterglow of GRB 120729A have the same external shock physical origin. Our model indicates that the ϵ B evolution can be described as a broken power-law function with α B ,1  = 0.18 ± 0.04 and α B ,2  = 0.84 ± 0.04. We also systematically investigate single-pulse GRBs in the Swift era, finding that only a small fraction of GRBs (GRBs 120729A, 051111, and 070318) are likely to originate from an external shock for both the prompt γ -ray emission and afterglow.