Spectropolarimetry and photometry of the early afterglow of the gamma-ray burst GRB 191221B

D. A.H. Buckley, S. Bagnulo, R. J. Britto, J. Mao, D. A. Kann, J. Cooper, V. Lipunov, D. M. Hewitt, S. Razzaque, N. P.M. Kuin, I. M. Monageng, S. Covino, P. Jakobsson, A. J. Van Der Horst, K. Wiersema, M. Böttcher, S. Campana, V. D'elia, E. S. Gorbovskoy, I. GorbunovD. N. Groenewald, D. H. Hartmann, V. G. Kornilov, C. G. Mundell, R. Podesta, J. K. Thomas, N. Tyurina, D. Vlasenko, B. Van Soelen, D. Xu

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11 Citations (Scopus)


We report on results of spectropolarimetry of the afterglow of the long gamma-ray burst GRB 191221B, obtained with SALT/RSS and VLT/FORS2, as well as photometry from two telescopes in the MASTER Global Robotic Network, at the MASTER-SAAO (South Africa) and MASTER-OAFA (Argentina) stations. Prompt optical emission was detected by MASTER-SAAO 38 s after the alert, which dimmed from a magnitude (white-light) of ∼10-16.2 mag over a period of ∼10 ks, followed by a plateau phase lasting ∼10 ks and then a decline to ∼18 mag after 80 ks. The light curve shows complex structure, with four or five distinct breaks in the power-law decline rate. SALT/RSS linear spectropolarimetry of the afterglow began ∼2.9 h after the burst, during the early part of the plateau phase of the light curve. Absorption lines seen at ∼6010 and 5490 Å are identified with the Mg ii 2799 Å line from the host galaxy at z = 1.15 and an intervening system located at z = 0.96. The mean linear polarization measured over 3400-8000 Å was ∼1.5 per cent and the mean equatorial position angle (θ) was ∼65°. VLT/FORS2 spectropolarimetry was obtained ∼10 h post-burst, during a period of slow decline (α = -0.44), and the polarization was measured to be p = 1.2 per cent and θ = 60°. Two observations with the MeerKAT radio telescope, taken 30 and 444 d after the GRB trigger, detected radio emission from the host galaxy only. We interpret the light curve and polarization of this long GRB in terms of a slow-cooling forward shock.

Original languageEnglish
Pages (from-to)4621-4631
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 1 Sept 2021


  • gamma-ray bursts
  • high energy astrophysics
  • jets
  • magnetic fields
  • polarimetry
  • shocks

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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