An Afterglow Study of the "New Year's Burst" GRB 220101A

We present a detailed broadband afterglow study of GRB 220101A ($10^4\lesssimΔT\lesssim10^7$ s) combining multi-wavelength data from soft X-rays until 6 GHz. The afterglow light curves in both X-ray and optical show distinct steepening around $\sim9$ days, followed by a sharp post-break decay index of $\sim2.99\pm0.10$. We fit the light curves using the afterglow modelling package \texttt{afterglowpy} for both Top-hat and Gaussian jets for different values of the electronic participation fraction $ξ$ from 0.01 to 1.0 and find that, although the radio behavior is well described by the $ξ=1.0$ case, the required circumburst medium (CBM) densities are very low, $1$ cm$^{-3}$) of GRB environments, which are highly star forming regions. Similarly low ISM densities have been seen in modeling of other LAT GRBs as well, especially ones with reverse-shock features (e.g., GRBs 130427A, 160509A and 160625B), thereby hinting at either an issue with the standard model or possible evacuated cavities where GRBs explode.