First alma light curve constrains refreshed reverse shocks & jet magnetization on 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


We present detailed multi-wavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first ALMA light curve of a GRB afterglow. The cm- and mm-band observations before 8.5 d 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, n0 ≈ 3 × 10−4 cm−3, and show that this is a characteristic of all strong RS detections to date. The VLA observations exhibit unexpected rapid variability on ∼ minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, tjet ≈ 32 d, yielding a wide jet opening angle of θjet ≈ 13, implying beaming corrected γ-ray and kinetic energies of Eγ ≈ 4.9 × 1048 erg and EK ≈ 1.3 × 1050 erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, RB ≈ 1, compared to the FS. These direct, multi-frequency 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 (US)
JournalUnknown Journal
StatePublished - Aug 28 2018


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

ASJC Scopus subject areas

  • General

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