2016ApJ...833...88L

We present the second multi-frequency radio detection of a reverse shock in a γ-ray burst. By combining our extensive radio observations of the Fermi-Large Area Telescope γ-ray burst 160509A at z = 1.17 up to 20 days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at <=10 days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of n₀~10^–3cm^–3, supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, N_H ≃ 1.5 x 10²² cm^–2, and a high rest-frame optical extinction, A_V ≃ 3.4 mag. We identify a jet break in the X-ray light curve at t_jet~6 days, and thus derive a jet opening angle of θ_jet~4^○, yielding a beaming-corrected kinetic energy and radiated γ-ray energy of E_K~4×10⁵⁰ erg and E_γ~1.3×10⁵¹ erg (1-10⁴ keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of t_dec ~460 s ≃ T₉₀, a Lorentz factor of Γ(t_dec)~330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of R_B≡ε_B,RS/ε_B,FS~8. Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of γ-ray burst ejecta.