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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 curveof a GRB afterglow. The cm- and mm-band observations before $8.5$ d requireemission in excess of that produced by the afterglow forward shock (FS). Thesedata are consistent with radiation from a refreshed reverse shock (RS) producedby the injection of energy into the FS, signatures of which are also present inthe X-ray and optical light curves. We infer a constant-density circumburstenvironment with an extremely low density, $n_0\approx 3\times10^{-4}$cm$^{-3}$ and show that this is a characteristic of all strong RS detections todate. The VLA observations exhibit unexpected rapid variability on $\sim$minute timescales, indicative of strong interstellar scintillation. The X-ray,ALMA, and VLA observations together constrain the jet break time, $t_{\rmjet}\approx32$ day, yielding a wide jet opening angle of $\theta_{\rmjet}\approx13^{\circ}$, implying beaming corrected $\gamma$-ray and kineticenergies of $E_{\gamma}\approx4.9\times10^{48}$ erg and $E_{\rmK}\approx1.3\times10^{50}$ erg, respectively. Comparing the RS and FS emission,we show that the ejecta are only weakly magnetized, with relativemagnetization, $R_{\rm B}\approx1$, compared to the FS. These direct,multi-frequency measurements of a refreshed RS spanning the optical to radiobands highlight the impact of radio and millimeter data in probing theproduction and nature of GRB jets.

First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B