Possible origin of AT2021any: A failed gamma-ray burst from a structured jet

Abstract

Afterglows not associated with any gamma-ray bursts (GRBs) are called orphan afterglows. The detection of such afterglows is an important goal in many sky survey programs. Recently, a promising orphan afterglow candidate, AT2021any, was found by the Zwicky Transient Facility. In this work, we performed multi-wavelength fittings of AT2021any with two different outflow models, namely, the top-hat jet model and the structured Gaussian jet model. Although both models can fit the observed light curves well, we found that the structured Gaussian jet model presents a better result and is thus preferred by observations. In this framework, the best-fit Lorentz factor is about 68, which indicates that AT2021any should be a failed GRB. The half-opening angle of the jet and the viewing angle were found to be 0.1 and 0.02, respectively, which means that the jet is essentially observed on-axis. We inferred the trigger time of the GRB to be about 1000 s before the first detection of the orphan afterglow, and we derived an upper limit of 21.5% for the radiative efficiency, which is typical for GRBs.