An Unexpectedly Small Emission Region Size Inferred from Strong High-Frequency Diffractive Scintillation in GRB 161219B

We present Karl G. Jansky Very Large Array radio observations of the long gamma-ray burst GRB 161219B ($z=0.147$) spanning $1-37$ GHz. The data exhibit unusual behavior, including sharp spectral peaks and minutes-timescale large-amplitude variability centered at $20$ GHz and spanning the full frequency range. We attribute this behavior to scattering of the radio emission by the turbulent ionized Galactic interstellar medium (ISM), including both diffractive and refractive scintillation. However, the scintillation is much stronger than predicted by a model of the Galactic electron density distribution (NE2001); from the measured variability timescale and decorrelation bandwidth we infer a scattering measure of $SM\approx {(8-70)\times 10^{-4}}$ kpc m$^{-20/3}$ (up to ${25}$ times larger than predicted in NE2001) and a scattering screen distance of $d_{\rm scr}\approx {0.2-3}$ kpc. We infer an emission region size of $\theta_s \approx {0.9-4}$ $\mu$as ($\approx {(1-4)}\times 10^{16}$ cm) at $\approx4$ days, and find that prior to 8 days the source size is an order of magnitude smaller than model predictions for a uniformly illuminated disk or limb-brightened ring, indicating a slightly off-axis viewing angle or significant substructure in the emission region. Simultaneous multi-hour broadband radio observations of future GRB afterglows will allow us to characterize the scintillation more completely, and hence to probe the observer viewing angle, the evolution of the jet Lorentz factor, the structure of the afterglow emission regions, and ISM turbulence at high Galactic latitudes.