Multiwavelength obs. of GRB 161219B

The centimeter- and millimeter-band observations before 8.5 days 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, n_0_~3x10^-4^cm^-3^, and show that this is a characteristic of all strong RS detections to date. The Karl G. Lansky Very Large Array (VLA) observations exhibit unexpected rapid variability on roughly minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, t_jet_~32 days, yielding a wide jet opening angle of {theta}_jet_~13{deg}, implying beaming-corrected {gamma}-ray and kinetic energies of E_{gamma}~4.9x10^48^erg and E_K_~1.3x10^50^erg, respectively.