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Register a new userGRB 140423A: A Case of Stellar Wind to Interstellar Medium Transition in the Afterglow
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We present very early ground-based optical follow-up observations of GRB~140423A, which was discovered by emph{Swift}/BAT and by {it Fermi}/GBM. Its broadband afterglow was monitored by {it Swift}/XRT and ground-based optical telescopes from $T_0+$70.96~s to 4.8~d after the {it Swift}/BAT trigger. This is one more case of prompt optical emission observation. The temporal and spectral joint fit of the multiwavelength light curves of GRB 140423A reveals that achromatic behavior is consistent with the external shock model including a transition from a stellar wind to the interstellar medium (ISM) and energy injection. In terms of the optical light curves, there is an onset bump in the early afterglow with a rising index $alpha_{rm O,I} = -0.59 pm 0.04$ (peaking at $t_{rm peak}-T_0 approx 206$~s). It then decays with a steep index $alpha_{rm O,II} = 1.78 pm 0.03$, and shows a steeper to flatter transition with $alpha_{rm O,III} = 1.13 pm 0.03$ at around $T_0 + 5000$~s. The observed X-ray afterglow reflects an achromatic behavior, as does the optical light curve. There is no obvious evolution of the spectral energy distribution between the X-ray and optical afterglow, with an average value of the photon index $Gamma approx 1.95$. This transition is consistent with an external shock model having the circumburst medium transition from a wind to the ISM, by introducing a long-lasting energy injection with a Lorentz factor stratification of the ejecta. The best parameters from Monte Carlo Markov Chain fitting are $E_{rm K,iso} approx 2.14times10^{55}$ erg, $Gamma_0 approx 162$, $epsilon_e approx 0.02$, $epsilon_B approx 1.7times10^{-6}$, $A_ast approx 1.0$, $R_t approx 4.1times10^{17}$ cm, $n approx 11.0 rm cm^{-3}$, $L_0 approx 3.1times10^{52} rm erg s^{-1}$, $k approx 1.98$, $s approx 1.54$, and $theta_j > 0.3$ rad.
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