Optical and Near-infrared Observations of the Distant but Bright “New Year’s Burst” GRB 220101A

Abstract

Abstract High-redshift gamma-ray bursts (GRBs) are useful to probe the early Universe, but only a few candidates have been detected so far. Here, we report the optical and near-infrared observations of the afterglow of a relatively high-redshift event GRB 220101A, which was triggered on New Year’s Day of 2022, and therefore referred to as the “New Year’s burst.” With the optical spectra obtained by XL2.16/BFOSC and NOT/ALFOSC, we determine the redshift of the burst to be z = 4.615. We find that the optical afterglow of GRB 220101A is one of the most luminous ever detected. Based on our optical and near-infrared data, and combined with the X-ray observations, we perform a multiband fit with the Python package afterglowpy. The jet opening angle is constrained to ∼3.°4, which is consistent with the jet-break time at ∼0.7 day. We also determine the circumburst density of n 0 = 0.15 cm−3 and kinetic energy E K,iso = 3.5 × 1054 erg. In the prompt phase of the burst, we find a “mirror” feature in the lightcurve from 80 s to 120 s. The physical origin of such a mirror feature is unclear.