First alma light curve constrains refreshed reverse shocks & jet magnetization on GRB 161219B

We present detailed multi-wavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first ALMA light curve of a GRB afterglow. The cm- and mm-band observations before 8.5 d require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, n₀ ≈ 3 × 10⁻⁴ cm⁻³, and show that this is a characteristic of all strong RS detections to date. The VLA observations exhibit unexpected rapid variability on ∼ minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, t_jet ≈ 32 d, yielding a wide jet opening angle of θ_jet ≈ 13^◦, implying beaming corrected γ-ray and kinetic energies of E_γ ≈ 4.9 × 10⁴⁸ erg and E_K ≈ 1.3 × 10⁵⁰ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, R_B ≈ 1, compared to the FS. These direct, multi-frequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.