A Reverse Shock in GRB 160509A

Tanmoy Laskar, Kate D. Alexander, Edo Berger, Wen-fai Fong, Raffaella Margutti, Isaac Shivvers, Peter K. G. Williams, Drejc Kopac, Shiho Kobayashi, Carole Mundell, Andreja Gomboc, WeiKang Zheng, Karl M. Menten, Melissa L. Graham, Alexei V. Filippenko

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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 $\lesssim10~$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 $n_0\approx10^{-3}~$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, $N_{\rm H} \approx 1.5\times10^{22}~$cm$^{-2}$, and a high rest-frame optical extinction, $A_{\rm V}\approx3.4~$mag. We identify a jet break in the X-ray light curve at $t_{\rm jet}\approx6~$d, and thus derive a jet opening angle of $\theta_{\rm jet}\approx4~$deg, yielding a beaming-corrected kinetic energy and radiated $\gamma$-ray energy of $E_{\rm K}\approx4\times10^{50}~$erg and $E_{\gamma}\approx1.3\times10^{51}~$erg ($1$-$10^4~$keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of $t_{\rm dec} \approx 460~$s$~\approx T_{90}$, a Lorentz factor of $\Gamma(t_{\rm dec})\approx330$, and a reverse shock to forward shock fractional magnetic energy density ratio of $R_{\rm B}\equiv\epsilon_{\rm B,RS}/\epsilon_{\rm B,FS}\approx8$.
Original languageEnglish
Article number88
Number of pages8
JournalAstrophysical Journal
Volume833
Issue number1
DOIs
Publication statusPublished - 8 Dec 2016

Keywords

  • astro-ph.HE

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    Laskar, T., Alexander, K. D., Berger, E., Fong, W., Margutti, R., Shivvers, I., Williams, P. K. G., Kopac, D., Kobayashi, S., Mundell, C., Gomboc, A., Zheng, W., Menten, K. M., Graham, M. L., & Filippenko, A. V. (2016). A Reverse Shock in GRB 160509A. Astrophysical Journal, 833(1), [88]. https://doi.org/10.3847/1538-4357/833/1/88