A REVERSE SHOCK IN GRB 160509A

We present the second multi-frequency radio detection of a reverse shock in a gamma-ray burst. By combining our extensive radio observations of the Fermi-Large Area Telescope gamma-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. less than or similar to 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) approximate to 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 approximate to 1.5. x 10(22) cm(-2), and a high rest-frame optical extinction, A(V) approximate to 3.4 mag. We identify a jet break in the X-ray light curve at t(jet) approximate to 6 days, and thus derive a jet opening angle of theta(jet) approximate to 4 degrees, yielding a beaming-corrected kinetic energy and radiated gamma-ray energy of E-K approximate to 4 x 10(50) erg and E-gamma approximate to 1.3 x 10(51) erg ( 1-10(4) keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of t(dec) approximate to 460 s approximate to T-90, a Lorentz factor of Gamma( t(dec)) approximate to 330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of R-B equivalent to is an element of(B, RS)/is an element of(B, FS) approximate to 8. Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of gamma-ray burst ejecta.