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

We present a detailed broadband afterglow study of GRB 220101A ($10^4\lesssim\Delta T\lesssim10^7$ 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 $\xi$ from 0.01 to 1.0 and find that, although the radio behavior is well described by the $\xi=1.0$ case, the required circumburst medium (CBM) densities are very low, $<10^{-4}$ cm$^{-3}$. However, the resulting energy requirements are modest, $\sim10^{52}$ erg, with an electron energy distribution (EED) index $p\sim2.05$. Similar results are also obtained from an analytic model fit to the light curve, except the predicted $p$ is higher, $\sim2.40$. The observed post-break decay index of $2.99$ is at least 5$\sigma$ away from $p$, which is one of the steepest observed so far. We also find that when ignoring the radio observations, the CBM density is raised by a few orders of magnitude to $\sim0.01$ cm$^{-3}$ for $\xi=1.0$, still far from the expected ISM density ($>1$ 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.