Laskar, TanmoyAlexander, KateBerger, EdoFong, Wen-faiMargutti, RaffaellaShivvers, IsaacWilliams, Peter K. G.Kopač, DrejcKobayashi, ShihoMundell, CaroleGomboc, AndrejaZheng, WeiKangMenten, Karl M.Graham, Melissa L.Filippenko, Alexei V.2017-03-062016Laskar, Tanmoy, Kate D. Alexander, Edo Berger, Wen-fai Fong, Raffaella Margutti, Isaac Shivvers, Peter K. G. Williams, et al. 2016. A Reverse Shock in Grb 160509a. The Astrophysical Journal 833, no. 1: 88. doi:10.3847/1538-4357/833/1/88.1538-4357http://nrs.harvard.edu/urn-3:HUL.InstRepos:30510299We 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-LAT GRB 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 ≈ 10−3 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, NH ≈ 1.5 × 1022 cm−2, and a high rest-frame optical extinction, AV ≈ 3.4 mag. We identify a jet break in the X-ray light curve at tjet ≈ 6 d, and thus derive a jet opening angle of θjet ≈ 4 ◦, yielding a beaming corrected kinetic energy and radiated γ-ray energy of EK ≈ 4 × 1050 erg and Eγ ≈ 1.3 × 1051 erg (1–104 keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of tdec ≈ 460 s ≈ T90, a Lorentz factor of Γ(tdec) ≈ 330, and a reverse shock to forward shock fractional magnetic energy density ratio of RB ≡ B,RS/ B,FS ≈ 8. Subject headings: gamma-ray burst: general – gamma-ray burst: individual (GRB 160509A)https://dash.harvard.edu/admin/epeopleen-USgamma-ray burst: generalgamma-ray burst: individualJournal Article2017-03-0610.3847/1538-4357/833/1/88