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

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