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 multi-wavelength afterglow light curves of recent GRBs indeed shows evidence for 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 broad-band spectrum alone. Such degeneracies can be lifted with joint modeling of the afterglow light curves and polarization. In this work we study 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 explore 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 demonstrate how our results can be used to determine the nature of orphan GRB afterglows, distinguishing between a quasi-spherical “dirty fireball” and a steep jets viewed far off-axis and apply them on the Zwicky Transient Facility (ZTF) detected orphan afterglow candidate AT2021lfa.

💡 Research Summary

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This paper investigates the linear polarization signatures of afterglow emission from steep, core‑dominated gamma‑ray burst (GRB) jets, where most of the kinetic energy is confined within a narrow core (θ_c ≈ 2°) and the energy falls off as a power‑law with index a > 2. The authors adopt the formalism of Birenbaum et al. (2024), modelling a relativistic blast wave propagating into a uniform interstellar medium (ρ ∝ R⁰). The jet’s isotropic‑equivalent kinetic energy is described by E_iso(θ) = E_c · Θ⁻ᵃ with Θ =