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

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 polarisation is expected at early and late phases depending on the presence of reverse shocks (RS) and the observer's viewing geometry relative to the jet. Polarimetric observations of GRB afterglows provide a unique diagnostic tool to probe the geometry and structure of magnetic fields in the emitting region, which cannot be inferred from photometric or spectroscopic data alone. We report late-time (~19 hours post-burst) spectropolarimetric observations of GRB 250129A using the Southern African Large Telescope (SALT). The data reveal a hint of linear polarisation, with no evidence for rotation in the polarisation angle across wavelengths. Polarisation is typically expected during the early afterglow (<100 s) when the RS dominates. However, multi-wavelength modelling shows no indication of RS contribution at late times. Modelling incorporating both forward shock (FS) and RS components confirms that the RS fades rapidly after ~100 s. The afterglow emission is best explained by an off-axis viewing geometry of a jet with a Gaussian core and wings evolving in a uniform density environment. GRB 250129A thus provides rare observational evidence linking late-time polarisation to jet geometry and structure.

Figures (10)

• Figure 1: Finding chart of GRB 250129A field (the CCD-photometer FoV placed ontop of central square of MAGIC FoV). Positions of OT and used standard stars are marked with the circles, their magnitudes are listed in the Table \ref{['standards']}
• Figure 2: Optical spectrum of GRB 250129A using SALT and DOT which is converted to the rest-frame wavelength. The DOT spectrum is indicated with red whereas the SALT spectrum is colured in blue. Spectral lines at different wavelengths are also indicated. Note that the RSS spectra have been binned with 50Å bins.
• Figure 3: Multi-wavelength light-curve modelling of GRB 250129A using a top-hat jet with combined FS+RS components. The top panel shows the best-fit model for a constant-density ISM environment, while the bottom panel illustrates the fit for a wind-like medium. The dataset includes Swift-XRT X-ray observations and selected optical bands with the highest data coverage.
• Figure 4: The polarization of GRB 250129A (blue) compared to a comparison star (orange) observed by SALT $\sim$ 19 hours since the burst. The plot shows the unbinned counts (top panel), and the degree of polarization (middle panel), and the position angle (bottom panel) binned on 500 Å. The middle panel shows the error in the mean value, and the standard deviation of the polarization measurements between 4500 and 7000 Å as shaded regions (see text for details). In the bottom panel the solid lines show the box car average of the polarization averaged over 10 data points.
• Figure 5: Light curves of GRB 250129A fitted with phenomenological single power-law model. Blue circles show the X-ray flux at 10 keV, while green and orange circles correspond to the optical V and r bands, respectively. Solid lines denote the best-fit models, with an index of $\alpha_X = 0.903 \pm 0.021$ for X-ray and $\alpha_o = 0.792 \pm 0.002$ for the optical bands.