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 _ { 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 \approx 4 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 .