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Full list of authors: Buckley, D. A. H.; Bagnulo, S.; Britto, R. J.; Mao, J.; Kann, D. A.; Cooper, J.; Lipunov, V.; Hewitt, D. M.; Razzaque, S.; Kuin, N. P. M.; Monageng, I. M.; Covino, S.; Jakobsson, P.; van der Horst, A. J.; Wiersema, K.; Böttcher, M.; Campana, S.; D'Elia, V.; Gorbovskoy, E. S.; Gorbunov, I.; Groenewald, D. N.; Hartmann, D. H.; Kornilov, V. G.; Mundell, C. G.; Podesta, R.; Thomas, J. K.; Tyurina, N.; Vlasenko, D.; van Soelen, B.; Xu, D. ; 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. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. ; Some of the observations presented here were obtained with SALT under programme 2018-2-LSP-001 (PI: DAHB), which is supported by Poland under grant no. MNiSW DIR/WK/2016/07. Based on observations collected at the European Southern Observatory under ESO programme 0104.D-0600(C). DAHB and JT acknowledge support through the National Research Foundation (NRF) of South Africa. MB is supported by the South African Research Chairs Initiative (grant no. 64789) of the Department of Science and Innovation and the NRF.5 DMH acknowledges financial support from the NRF and the SAAO. SR is partially supported by NRF with grant no. 111749 (CPRR) and by a University of Johannesburg Research Council grant. DAK acknowledges support from Spanish National Research Project RTI2018-098104-J-I00 (GRBPhot). NPMK acknowledges support by the UK Space Agency. MASTER (equipment) is supported by Lomonosov Moscow State University Development Program. VL and DV are supported by RFBR grant 19-29-11011. CGM acknowledges financial support from Hiroko and Jim Sherwin. We thank the Director and staff of SARAO for supporting our MeerKAT DDT observation. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. Based on observations made with the SALT and the MeerKAT radio telescope array. ; With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709. ; Peer reviewed

Black holes in binary systems execute patterns of outburst activity where two characteristic X-ray states are associated with different behaviours observed at radio wavelengths. The hard state is associated with radio emission indicative of a continuously replenished, collimated, relativistic jet, whereas the soft state is rarely associated with radio emission, and never continuously, implying the absence of a quasi-steady jet. Here we report radio observations of the black hole transient MAXI J1820+070 during its 2018 outburst. As the black hole transitioned from the hard to soft state, we observed an isolated radio flare, which, using high-angular-resolution radio observations, we connect with the launch of bipolar relativistic ejecta. This flare occurs as the radio emission of the core jet is suppressed by a factor of over 800. We monitor the evolution of the ejecta over 200 days and to a maximum separation of 10″, during which period it remains detectable due to in situ particle acceleration. Using simultaneous radio observations sensitive to different angular scales, we calculate an accurate estimate of energy content of the approaching ejection. This energy estimate is far larger than that derived from the state transition radio flare, suggesting a systematic underestimate of jet energetics. © 2020, The Author(s), under exclusive licence to Springer Nature Limited. ; J.S.B. acknowledges the support of a Science and Technologies Facilities Council Studentship. E.T. acknowledges financial support from the UnivEarthS Labex programme of Sorbonne Paris Cite (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). D.A.H.B. acknowledges support by the National Research Foundation. P.A.W. acknowledges support from the NRF and UCT. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT140101082), funded by the Australian government. A.H. acknowledges that this research was supported by a grant from the GIF, the German-Israeli Foundation for Scientific Research and Development. I.H. and D.R.A.W. acknowledge support from the Oxford Hintze Centre for Astrophysical Surveys, which is funded through generous support from the Hintze Family Charitable Foundation. J.M. acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709) and from the grant RTI2018-096228-B-C31 (MICIU/FEDER, EU). The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Technology. We thank the staff of the Mullard Radio Astronomy Observatory for their invaluable assistance in the commissioning, maintenance and operation of AMI, which is supported by the universities of Cambridge and Oxford. We acknowledge support from the European Research Council under grant ERC-2012-StG-307215 LODESTONE. We thank the Swift team for performing observations promptly on short notice. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. We acknowledge the use of the Inter-University Institute for Data Intensive Astronomy (IDIA) data-intensive research cloud for data processing. IDIA is a South African university partnership involving the University of Cape Town, the University of Pretoria and the University of the Western Cape. We thank the International Space Science Institute in Bern, Switzerland for support and hospitality for the team meeting 'Looking at the disc-jet coupling from different angles: inclination dependence of black-hole accretion observables'. ; Peer reviewed