2020ApJ...900..176L

We present very early ground-based optical follow-up observations of gamma-ray burst (GRB) 140423A, which was discovered by the Swift/Burst Alert Telescope (BAT) and by the Fermi/Gamma-Ray Burst Monitor. Its broadband afterglow was monitored by the Swift/X-Ray Telescope and ground-based optical telescopes from T₀ + 70.96 s to 4.8 days after the Swift/BAT trigger. This is one more case of a prompt optical emission observation. The temporal and spectral joint fits of the multiwavelength light curves of GRB 140423A reveal 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 α_O,I=-0.59±0.04 (peaking at t_peak-T₀~206 s). It then decays with a steep index α_O,II=1.78±0.03, and shows a steeper to flatter "transition" with α_O,III=1.13±0.03 at around T₀ + 5000 s. The observed X-ray afterglow reflects achromatic behavior, as does the optical light curve. There is no obvious evolution of the spectral energy distribution between the X-ray and optical afterglows, with an average value of the photon index Γ~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_K,iso~2.14×10⁵⁵ erg, Γ₀~162, ε_e~0.02, ε_B~1.7×10^–6, A_*~1.0, R_t~4.1×10¹⁷ cm, n~11.0cm^–3, L₀~3.1×10⁵²erg s^–1, k~1.98, s~1.54, and θ_j> 0.3rad.