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

Tanmoy Laskar, Kate D. Alexander, Edo Berger, Cristiano Guidorzi, Raffaella Margutti, Wen-Fai Fong, Charles D. Kilpatrick, Peter Milne, Maria R. Drout, C. G. Mundell, Shiho Kobayashi, Ragnhild Lunnan, Rodolfo Barniol Duran, Karl M. Menten, Kunihito Ioka, Peter K. G. Williams

Research output: Contribution to journalArticlepeer-review

33 Citations (SciVal)


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, , 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, days, yielding a wide jet opening angle of , implying beaming-corrected γ-ray and kinetic energies of erg and erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, , 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.

Original languageEnglish
Article number94
Pages (from-to)1-23
Number of pages23
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 26 Jul 2018


  • gamma-ray burst: general
  • gamma-ray burst: individual (GRB 161219B)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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