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 , n _ { 0 } \approx 3 \times 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 \sim minute timescales , indicative of strong interstellar scintillation .
The X-ray , ALMA , and VLA observations together constrain the jet break time , t _ { jet } \approx 32 d , yielding a wide jet opening angle of \theta _ { jet } \approx 13 ^ { \circ } , implying beaming corrected \gamma -ray and kinetic energies of E _ { \gamma } \approx 4.9 \times 10 ^ { 48 } erg and E _ { K } \approx 1.3 \times 10 ^ { 50 } erg , respectively .
Comparing the RS and FS emission , we show that the ejecta are only weakly magnetized , with relative magnetization , R _ { B } \approx 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 .