Inferring Jet Structure and Viewing Angles of Gamma-Ray Burst Jets from Early Rising Optical Afterglows

Abstract We present the first systematic investigation of off-axis jets in gamma-ray bursts based on rising optical afterglows. As a pilot study, we selected eight well-observed bright optical afterglows and performed a Bayesian inference using two different jet models: a Gaussian-structured jet and a homogeneous top-hat jet. Assuming an adiabatic evolution of the fireball through a constant-density ambient medium, we derived the key afterglow parameters: viewing angle ( θ v ), jet opening angle ( θ j for top-hat jets) or characteristic angle ( θ c for Gaussian jets), ambient medium number density n 0 , and ϵ B measuring the downstream magnetic field strength. The inferred viewing angles of the sample range from 0.4° to 9.7°. Our results identify delayed optical peaks (occurring hundreds to thousands of seconds postburst) and low luminosity as key signatures of misaligned jets, with slow rises to peak suggesting structured jets. We further observe a correlation between the rest-frame peak time and the degree of misalignment in optical light curves. A larger sample size is required to confirm this.