Afterglow linear polarization signatures from steep GRB jets: Implications for orphan afterglows

Gamma-ray bursts (GRBs) are the strongest explosions in the Universe, and are powered by initially ultra-relativistic jets. The angular profile of GRB jets encodes important information about their launching and propagation near the central source, and can be probed through their afterglow emission. Detailed analysis of the multiwavelength afterglow light curves of recent GRBs shows evidence of an extended angular structure beyond the jet’s narrow core. The afterglow emission is determined by the jet angular structure, our viewing angle, and the magnetic field structure behind the shock, often leading to degeneracies when considering the light curves and broadband spectrum alone. Such degeneracies can be lifted with joint modeling of the afterglow light curves and polarization. In this work we studied the evolution of the afterglow linear polarization and flux density from steep core-dominated GRB jets, where most of their energy resides within a narrow core. We explored the dependence of the light and polarization curves on the viewing angle, jet angular energy structure, and magnetic field configuration and provide an analytical approximation for the peak polarization level, which occurs at a time close to that of a break in the light curve. Finally, we demonstrated how our results can be used to determine the nature of orphan GRB afterglows, distinguishing between a quasi-spherical dirty fireball and a steep jet viewed far off-axis, and applied them to the orphan afterglow candidate AT2021lfa detected by the Zwicky Transient Facility (ZTF).