A Reverse Shock in GRB 160509A

Abstract

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-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 $\lesssim 10$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 $n_0\approx 10^{-3}$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, $N_{\mathrm{H}}\approx 1.5\times 10^{22}$cm${}^{-2}$, and a high rest-frame optical extinction, $A_{\mathrm{V}}\approx 3.4$mag. We identify a jet break in the X-ray light curve at $t_{\mathrm{jet}}\approx 6$d, and thus derive a jet opening angle of ${\theta }_{\mathrm{jet}}\approx 4$deg, yielding a beaming-corrected kinetic energy and radiated $\gamma$-ray energy of $E_{\mathrm{K}}\approx 4\times 10^{50}$erg and $E_{\gamma }\approx 1.3\times 10^{51}$erg ($1$-$10^4$keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of $t_{\mathrm{dec}}\approx 460$s$\approx T_{90}$, a Lorentz factor of $\Gamma (t_{\mathrm{dec}})\approx 330$, and a reverse shock to forward shock fractional magnetic energy density ratio of $R_{\mathrm{B}}\equiv {\epsilon }_{\mathrm{B},\mathrm{RS}}/{\epsilon }_{\mathrm{B},\mathrm{FS}}\approx 8$.

8 pages, 2 tables, 4 figures. Submitted to ApJ

References in corpus (6)

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• Multiwavelength analysis of the intriguing GRB 061126: the reverse shock scenario and magnetization
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• A robotic pipeline for fast GRB followup with the Las Cumbres observatory network