Multiple Rebrightenings in the Optical Afterglow of GRB 210731A: Evidence for an Asymmetric Jet

Abstract The broadband afterglow of gamma-ray bursts (GRBs) is usually believed to originate from the synchrotron radiation of electrons accelerated by the external shock of relativistic jets. Therefore, the jet structure should have a significant impact on the GRB afterglow features. The latest observations indicate that the GRB jets may possess intricate structures, such as Gaussian structure, power-law structure, or jet-cocoon structure. Most recently, an abnormal afterglow of GRB 210731A has raised extensive attention, whose optical afterglow exhibits multiple rebrightening phenomena within 4 hr, posing a serious challenge to the standard afterglow model. Here we intend to interpret the characteristics of GRB 210731A afterglows within the framework of nonaxisymmetric structured jets, where multiple distinct peaks in the afterglow light curve are caused by the uneven distribution of energy and velocity within the jet in the azimuth angle direction. Through Markov Chain Monte Carlo fitting, we show that a three-component asymmetric structured jet can well explain the multiband afterglow data. The energy difference among the three components is about 2.3 orders of magnitude, with higher-energy components exhibiting faster speeds. The radiation contribution of each component has sequentially dominated the light curve of the afterglow, resulting in multiple peaks, with the highest peak occurring at the latest time. We suggest that in the future, polarization observations should be conducted on afterglows with multiple brightening signatures, which will help to effectively distinguish the structured jet model from other alternative models, such as energy injection, and ultimately help to determine the true configuration of jets.