2025RAA....25b5002L

Multiband afterglow observations of gamma-ray bursts (GRBs) are important for studying the central engine. GRB 201223A is a GRB with prompt optical detection by GWAC. Here we report on the early optical afterglow of GRB 201223A detected by NEXT (only 2.8 minutes after the Swift/BAT trigger), which smoothly connects the prompt optical emission and the afterglow phase. Utilizing Amati diagrams and considering the detection of afterglow emission in the Swift u-band, we suggest a redshift range of 0.26-1.85. Based on our optical data and combined with early optical observation from GWAC and early X-ray data from Swift/XRT, a multiband fitting is performed using PyFRS, and we obtain the best afterglow parameters (assuming a redshift of z = 1.0): ${E}_{{\rm{K}},\mathrm{iso}}={5.01}_{-1.70}^{+1.91}\times {10}^{54}$ erg, ${{\rm{\Gamma }}}_{0}={426.58}_{-138.18}^{+148.86}$, ${\theta }_{{\rm{j}}}={25.98}_{-10.54}^{+9.67}$ deg, ${n}_{0}={0.30}_{-0.26}^{+3.78}$ cm^–3, $p={2.32}_{-0.01}^{+0.01}$, ${\epsilon }_{{\rm{e}}}={3.31}_{-0.86}^{+1.59}\times {10}^{-5}$, ${\epsilon }_{{\rm{B}}}={3.47}_{-2.62}^{+4.12}\times {10}^{-1}$. The late-time X-ray shows a re-brightening, indicating late-time central engine activities. After comparing the leading two central engine models, i.e., magnetar model and hyperaccreting black hole model, we find that the fallback accretion onto a newborn black hole provides a better explanation for the X-ray re-brightening with fallback accretion rate $\dot{{M}_{{\rm{p}}}}\simeq 2.76\times {10}^{-9}{M}_{\odot }\,{{\rm{s}}}^{-1}$ and the total fallback accreted mass M_fb ≃ 1.41 × 10^–6M_⊙.