GRB201223A: Implication of Fallback Accretion onto the Newborn Black Hole from its Multiband Afterglow

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 min 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, early X-ray data from Swift/XRT, a multiband fitting is performed using PyFRS, and obtain the best afterglow parameters (assuming a redshift of $z=1.0$): EK,iso=5.01-1.70+1.91× 10⁵⁴ erg, Γ₀=426.58-138.18+148.86, θⱼ=25.98-10.54+9.67 deg, n₀=0.30-0.26+3.78 cm⁻³, p=2.32-0.01+0.01, εₑ=3.31-0.86+1.59× 10⁻⁵, εB=3.47-2.62+4.12× 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 finds that the fallback accretion onto a newborn black hole provides a better explanation for the X-ray re-brightening with fallback accretion rate Mₚ 2.76 × 10⁻⁹M_ s⁻¹ and the total fallback accreted mass Mfb 1.41 × 10⁻⁶M_.