A Reverse Shock in GRB 160509A

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 {n}₀≈ {10}^-3 {{cm}}^-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 × 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^\circ , yielding a beaming-corrected kinetic energy and radiated γ-ray energy of {E}_{{K}}≈ 4× {10}⁵⁰ erg and {E}_γ ≈ 1.3× {10}⁵¹ 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 ≈ T ₉₀, a Lorentz factor of {{Γ }}({t}_{dec})≈ 330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of {R}_{{B}}\equiv {ɛ }_{{B},{RS}}/{ɛ }_{{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.