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We present multi-wavelength observations and modeling of the exceptionallybright long $\gamma$-ray burst GRB 160625B. The optical and X-ray data arewell-fit by synchrotron emission from a collimated blastwave with an openingangle of $\theta_j\approx 3.6^\circ$ and kinetic energy of $E_K\approx2\times10^{51}$ erg, propagating into a low density ($n\approx 5\times10^{-5}$cm$^{-3}$) medium with a uniform profile. The forward shock is sub-dominant inthe radio band; instead, the radio emission is dominated by two additionalcomponents. The first component is consistent with emission from a reverseshock, indicating an initial Lorentz factor of $\Gamma_0\gtrsim 100$ and anejecta magnetization of $R_B\approx 1-100$. The second component exhibitspeculiar spectral and temporal evolution and is most likely the result ofscattering of the radio emission by the turbulent Milky Way interstellar medium(ISM). Such scattering is expected in any sufficiently compact extragalacticsource and has been seen in GRBs before, but the large amplitude and longduration of the variability seen here are qualitatively more similar to extremescattering events previously observed in quasars, rather than normalinterstellar scintillation effects. High-cadence, broadband radio observationsof future GRBs are needed to fully characterize such effects, which cansensitively probe the properties of the ISM and must be taken into accountbefore variability intrinsic to the GRB can be interpreted correctly.

A Reverse Shock and Unusual Radio Properties in GRB 160625B