The triple-peaked afterglow of GRB 210731A from X-ray to radio frequencies^⋆

¹ Inter-University Institute for Data Intensive Astronomy & Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
e-mail: DWTSIM002@myuct.ac.za
² Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112, USA
³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁴ South African Astronomical Observatory, PO Box 9 7935 Observatory, South Africa
⁵ INAF Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
⁸ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
⁹ Cosmic Dawn Center (DAWN), Denmark
¹⁰ Niels Bohr Institute, University of Copenhagen, Rådmandsgade 62-64, 2200 Copenhagen N, Denmark
¹¹ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹² Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Strasse 12, Goethe University Frankfurt, Campus Riedberg, 60438 Frankfurt am Main, Germany
¹³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
¹⁴ Department of Astronomy, School of Physics, Huazhong University of Science and Technology, Wuhan 430074, PR China

Received: 7 September 2022
Accepted: 23 January 2023

Abstract

Context. GRB 210731A was a long-duration (T₉₀ = 22.5 s) gamma-ray burst discovered by the Burst Alert Telescope (BAT) aboard the Neil Gehrels Swift Observatory. Swift triggered the wide-field, robotic MeerLICHT optical telescope in Sutherland; it began observing the BAT error circle 286 s after the Swift trigger and discovered the optical afterglow of GRB 210731A in its first 60-s q-band exposure. Multi-colour observations of the afterglow with MeerLICHT revealed a light curve that showed three peaks of similar brightness within the first four hours. The unusual optical evolution prompted multi-wavelength follow-up observations that spanned from X-ray to radio frequencies.

Aims. We present the results of our follow-up campaign and interpret our observations in the framework of the synchrotron forward shock model.

Methods. We performed temporal and spectral fits to determine the spectral regime and external medium density profile, and performed detailed multi-wavelength theoretical modelling of the afterglow following the last optical peak at ∼0.2 days to determine the intrinsic blast wave parameters.

Results. We find a preference for a stellar wind density profile consistent with a massive star origin, while our theoretical modelling results in fairly typical shock microphysics parameters. Based on the energy released in γ rays and the kinetic energy in the blast wave, we determine a low radiative efficiency of η ≈ 0.02. The first peak in the optical light curve is likely the onset of the afterglow. We find that energy injection into the forward shock offers the simplest explanation for the subsequent light curve evolution, and that the blast wave kinetic energy increasing by a factor of ∼1000 from the first peak to the last peak is indicative of substantial energy injection. Our highest-likelihood theoretical model over-predicts the 1.4 GHz flux by a factor of approximately three with respect to our upper limits, possibly implying a population of thermal electrons within the shocked region.