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First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B

Laskar, T, Alexander, KD, Berger, E, Guidorzi, C, Margutti, R, Fong, W-F, Kilpatrick, CD, Milne, P, Drout, MR, Mundell, CG, Kobayashi, S, Lunnan, R, Duran, RB, Menten, KM, Ioka, K and Williams, PKG (2018) First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B. Astrophysical Journal, 862 (2). ISSN 0004-637X

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Abstract

We present detailed multiwavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first Atacama Large Millimeter/submillimeter Array (ALMA) light curve of a γ-ray burst (GRB) afterglow. The centimeter- and millimeter-band observations before 8.5 days require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, ${n}_{0}\approx 3\times {10}^{-4}\,{\mathrm{cm}}^{-3}$, and show that this is a characteristic of all strong RS detections to date. The Karl G. Lansky Very Large Array (VLA) observations exhibit unexpected rapid variability on roughly minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, ${t}_{\mathrm{jet}}\approx 32$ days, yielding a wide jet opening angle of ${\theta }_{\mathrm{jet}}\approx 13^\circ $, implying beaming-corrected γ-ray and kinetic energies of ${E}_{\gamma }\approx 4.9\times {10}^{48}$ erg and ${E}_{{\rm{K}}}\approx 1.3\times {10}^{50}$ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, ${R}_{{\rm{B}}}\approx 1$, compared to the FS. These direct, multifrequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society; IOP Publishing
Related URLs:
Date Deposited: 17 Oct 2018 08:09
Last Modified: 18 Oct 2018 14:12
DOI or Identification number: 10.3847/1538-4357/aacbcc
URI: http://researchonline.ljmu.ac.uk/id/eprint/9490

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