AGILE Observations of GRB 220101A: A "New Year's Burst" with an Exceptionally Huge Energy Release

TL;DR

This paper reports on AGILE's observations of the exceptionally energetic GRB 220101A, detailing its spectral evolution, high isotropic energy, and afterglow analysis, highlighting its significance among gamma-ray bursts.

Contribution

The study provides the first detailed spectral and temporal analysis of GRB 220101A across a wide energy range, including its afterglow, and models its environment as wind-like.

Findings

GRB 220101A has an isotropic energy of 2.54x10^54 erg.

Spectral hardening observed during the burst's central phase.

Afterglow analysis suggests a wind-like surrounding medium.

Abstract

We report the AGILE observations of GRB 220101A, which took place at the beginning of 1st January 2022 and was recognized as one of the most energetic gamma-ray bursts (GRBs) ever detected since their discovery. The AGILE satellite acquired interesting data concerning the prompt phase of this burst, providing an overall temporal and spectral description of the event in a wide energy range, from tens of keV to tens of MeV. Dividing the prompt emission into three main intervals, we notice an interesting spectral evolution, featuring a notable hardening of the spectrum in the central part of the burst. The average fluxes encountered in the different time intervals are relatively moderate, with respect to those of other remarkable bursts, and the overall fluence exhibits a quite ordinary value among the GRBs detected by MCAL. However, GRB 220101A is the second farthest event detected by AGILE, and the burst with the highest isotropic equivalent energy of the whole MCAL GRB sample, releasing E_iso=2.54x10^54 erg and exhibiting an isotropic luminosity of L_iso=2.34x10^52 erg/s (both in the 400 keV - 10 MeV energy range). We also analyzed the first 10^6 s of the afterglow phase, using the publicly available Swift XRT data, carrying out a theoretical analysis of the afterglow, based on the forward shock model. We notice that GRB 220101A is with high probability surrounded with a wind-like density medium, and that the energy carried by the initial shock shall be a fraction of the total E_iso, presumably near 50%.