GRB 220408B: A Three-Episode Burst from a Precessing Jet

TL;DR

This paper models the complex three-episode light curve of GRB 220408B using a precessing jet with specific periods, explaining the observed temporal and spectral similarities.

Contribution

It applies a precession model to a bright GRB, demonstrating how jet precession can produce multi-episode light curves with consistent spectral features.

Findings

Precession period of 18.4 seconds and nutation period of 11.1 seconds identified.

Three emission episodes are explained by a precessing jet model.

Spectral evolution and lags are consistent with the precession hypothesis.

Abstract

Jet precession has previously been proposed to explain the apparently repeating features in the light curves of a few gamma-ray bursts (GRBs). In this {\it Letter}, we further apply the precession model to a bright GRB 220408B by examining both its temporal and spectral consistency with the predictions of the model. As one of the recently confirmed GRBs observed by our GRID CubeSat mission, GRB 220408B is noteworthy as it exhibits three apparently similar emission episodes. Furthermore, the similarities are reinforced by their strong temporal correlations and similar features in terms of spectral evolution and spectral lags. Our analysis demonstrates that these features can be well explained by the modulated emission of a Fast-Rise-Exponential-Decay (FRED) shape light curve intrinsically produced by a precessing jet with a precession period of $18.4 \pm 0.2$ seconds, a nutation period of $11.1 \pm 0.2$ seconds and viewed off-axis. This study provides a straightforward explanation for the complex yet similar multi-episode GRB light curves.