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

Abstract

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, $t_{\mathrm{jet}}\approx 32$ day, yielding a wide jet opening angle of $\hat{I}{¸}_{\mathrm{jet}}\approx 13^{\circ }$, implying beaming corrected ${\hat{I}}^3$-ray and kinetic energies of $E_{\hat{I}}^3\approx 4.9\times 10^{48}$ erg and $E_{\mathrm{K}}\approx 1.3\times 10^{50}$ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, $R_{\mathrm{B}}\approx 1$, 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.

23 pages, 17 figures, 9 tables. Published in ApJ