Work

Photometric and late-time spectropolarimetric observations of GRB 250129A afterglow

Year: 2026

Type: article

Abstract: ABSTRACT Gamma-ray burst (GRB) afterglows arise from the interaction of relativistic ejecta with the circumburst medium and are observed across the electromagnetic spectrum. Afterglow polarization is expected at early and late afterglow phases depending on the presence of reverse shocks (RS) and the observer’s viewing geometry relative to the jet. Polarimetric observations of GRB afterglows serve as a unique diagnostic tool to investigate the geometry and structure of magnetic fields in the emitting region, which cannot be directly inferred from photometric or spectroscopic data alone. We report late-time ($\sim$ 19 h post-burst) spectropolarimetric observations of GRB 250129A using the Southern African Large Telescope. The data reveal a hint of linear polarization, but rotation in polarization angle across wavelengths is not observed. Polarization is usually expected during the early afterglow ($\lesssim$ 100 s) when the RS dominates. However, multiwavelength modelling of GRB 250129A data shows no indication of RS contribution during the late epoch. Afterglow model incorporating both the forward shock and RS confirms that RS component fades rapidly after $\sim$100 s. The afterglow emission is best explained by an off-axis viewing geometry of a jet with Gaussian core and wing evolving in a uniform density environment. GRB 250129A thus provides a rare observational evidence linking late-time polarization to geometric and jet-structure effects.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag1391