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A Reverse Shock in GRB 160509A

Jun 28, 2016
8 pages
Published in:
  • Astrophys.J. 833 (2016) 1, 88
  • Published: Dec 8, 2016
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Abstract: (IOP)
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 n0≈10−3{n}_{0}\approx {10}^{-3} cm−3{\mathrm{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 × 10(22) cm−2{\mathrm{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 tjet≈6 days{t}_{\mathrm{jet}}\approx 6\,\mathrm{days}, and thus derive a jet opening angle of θjet≈4∘{\theta }_{\mathrm{jet}}\approx 4^\circ , yielding a beaming-corrected kinetic energy and radiated γ-ray energy of EK≈4×1050{E}_{{\rm{K}}}\approx 4\times {10}^{50} erg and Eγ≈1.3×1051{E}_{\gamma }\approx 1.3\times {10}^{51} erg (1–10(4) keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of tdec ≈460{t}_{\mathrm{dec}}\,\approx 460 s ≈ T (90), a Lorentz factor of Γ(tdec)≈330{\rm{\Gamma }}({t}_{\mathrm{dec}})\approx 330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of RB≡ϵB,RS/ϵB,FS≈8{R}_{{\rm{B}}}\equiv {\epsilon }_{{\rm{B}},\mathrm{RS}}/{\epsilon }_{{\rm{B}},\mathrm{FS}}\approx 8. Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of γ-ray burst ejecta.
Note:
  • 8 pages, 2 tables, 4 figures. Submitted to ApJ
  • gamma-ray burst: general
  • gamma-ray burst: individual
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