GRB 140423A: A Case of Stellar Wind to Interstellar Medium Transition in the Afterglow

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 ${\alpha }_{{\rm{O}},{\rm{I}}}=-0.59\pm 0.04$ (peaking at ${t}_{\mathrm{peak}}-{T}_{0}\approx 206$ s). It then decays with a steep index ${\alpha }_{{\rm{O}},\mathrm{II}}=1.78\pm 0.03$ , and shows a steeper to flatter "transition" with ${\alpha }_{{\rm{O}},\mathrm{III}}=1.13\pm 0.03$ 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 ${\rm{\Gamma }}\approx 1.95$ . 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 ${E}_{{\rm{K}},\mathrm{iso}}\approx 2.14\times {10}^{55}$ erg, ${{\rm{\Gamma }}}_{0}\approx 162$ , ${\varepsilon }_{e}\approx 0.02$ , ${\varepsilon }_{B}\approx 1.7\times {10}^{-6}$ , ${A}_{* }\approx 1.0$ , ${R}_{t}\approx 4.1\times {10}^{17}$ cm, $n\approx 11.0\,\,{\mathrm{cm}}^{-3}$ , ${L}_{0}\approx 3.1\times {10}^{52}\,\mathrm{erg}\ {{\rm{s}}}^{-1}$ , $k\approx 1.98$ , $s\,\approx 1.54$ , and ${\theta }_{j}\gt 0.3\,\mathrm{rad}$ .