From precursor to afterglow: The complex evolution of GRB 210312B

¹ Astronomical Institute of the Czech Academy of Sciences (ASU-CAS), Fričova 298, 251 65 Ondřejov, Czech Republic
² Space Research Institute of the Russian Academy of Sciences (IKI RAS), Profsoyuznaya 84/32, Moscow 117997, Russia
³ Artemis, Observatoire de la Côte d’Azur, CNRS, 06300 Nice, France
⁴ INAF–Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, I-40129 Bologna, Italy
⁵ Institute of Physics of Czech Academy of Sciences, Prague, Czech Republic
⁶ School of Physics & Astronomy, University of Birmingham, Birmingham B15 2TT, UK
⁷ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, Gérgal, Almería 04550, Spain
⁸ National Research University “Higher School of Economics”, Myasnitskaya 20, Moscow 101000, Russia
⁹ Faculty of Electrical Engineering – FEE-CTU – ČVUT, Prague, Czech Republic
¹⁰ INAF–Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio Catone, Italy
¹¹ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada, Spain

^⋆ Corresponding author: mates@asu.cas.cz

Received: 30 December 2024
Accepted: 14 April 2025

Abstract

Context. Long gamma-ray bursts (GRBs) are characterized by a brief gamma-ray flash followed by a longer-lasting multiwavelength afterglow. The basic mechanism is largely understood, and the early afterglow evolution often shows complex features that provide crucial insights into the transition between prompt and afterglow phases.

Aims. We present a detailed analysis of GRB 210312B, detected by INTEGRAL, which exhibits both a precursor and a complex optical afterglow evolution. Through careful modeling using Markov chain Monte Carlo methods, we disentangled the contributions of an early optical flare and forward shock emission.

Methods. Our analysis reveals a gamma-ray precursor 17 s before the main pulse with a significantly softer spectrum (hardness ratio 0.37±0.12 versus 1.9±0.4). The optical afterglow shows an early peak at 76.0_−5.1^+4.4 s characterized by a steep rise (α_flare,1 = −4.1_−2.3^+1.6) and decay (α_flare,2 = 4.0_−1.5^+2.1), followed by forward shock emission with a broad hydrodynamic peak at around 150 s. In the subsequent plateau phase, the afterglow initially has a complex structure before settling into a final power law decay consistent with an electron distribution index p = 2.36_−0.15^+0.18. The negligible host extinction (A_V,host = −0.073_−0.078^+0.100) suggests we are observing the intrinsic afterglow spectrum. The host system consists of two luminous (M_B∼−21.7) components separated by 11.5 kpc at z = 1.069, which are possibly an interacting galaxy pair.

Results. GRB 210312B provides a rare opportunity to study the prompt-to-afterglow transition in detail. The consistency of the forward shock component with standard afterglow theory supports our physical interpretation despite the lack of X-ray coverage.