A comprehensive broadband analysis of the high-redshift GRB 240218A

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
² Università degli Studi dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy
³ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁴ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, (RM), Italy
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁸ INAF – IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁹ Universidad Nacional Autonoma de Mexico, Instituto de Astronomia, Aptdo Postal 106, Ensenada, 22860 Baja California, Mexico
¹⁰ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, Gérgal, Almería 04550, Spain
¹¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹² Ingeniería de Sistemas y Automática, Universidad de Málaga, Unidad Asociada al CSIC por el IAA, Escuela de Ingenierías Industriales, Arquitecto Francisco Peñalosa, 6, Campanillas, 29071 Málaga, Spain
¹³ Space Science Data Center (SSDC) – Agenzia Spaziale Italiana (ASI), 00133 Roma, Italy
¹⁴ University of Messina, Mathematics, Informatics, Physics and Earth Science Department, Via F.S. D’Alcontres 31, Polo Papardo, 98166 Messina, Italy
¹⁵ Observatoire de la Côte d’Azur, Université Côte d’Azur, Boulevard de l’Observatoire, 06304 Nice, France
¹⁶ Aix Marseille Univ., CNRS, LAM, Marseille, France
¹⁷ University of Cape Town Astronomy Department, Private Bag X3, Rondebosch 7701, South Africa
¹⁸ Clemson University, Department of Physics and Astronomy, Clemson, SC 29634-0978, USA
¹⁹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
²⁰ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
²¹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
²² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
²³ Department of Astrophysics/IMAPP, Radboud University, 6525 AJ Nijmegen, The Netherlands
²⁴ School of Physics and Centre for Space Research, University College Dublin, Belfield, D04 V1W8 Dublin, Ireland
²⁵ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, India
²⁶ Astronomical Institute Anton Pannekoek, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
²⁷ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
²⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
²⁹ Dipartimento di Fisica, Università degli Studi di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³⁰ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jules Janssen, 92190 Meudon, France
³¹ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
³² Astronomical Institute, Czech Academy of Sciences, Fričova 298, Ondřejov, Czech Republic
³³ School of Mathematical and Physical Sciences, Macquarie University, NSW 2109, Australia

^⋆ Corresponding author; riccardo.brivio@inaf.it

Received: 25 October 2024
Accepted: 18 February 2025

Abstract

Context. The detection and follow-up observations of high-redshift (z > 6) gamma-ray bursts (GRBs) provide a unique opportunity to explore the properties of the distant Universe. Unfortunately, they are rather rare, with only a dozen of them identified so far.

Aims. We present here the discovery of the GRB with the second highest spectroscopic redshift measured to date, GRB 240218A at z = 6.782, and the broadband analysis of its afterglow. Following the detection by high-energy satellites, we obtained multi-epoch and multi-wavelength photometric follow-up observations, from 68 s to ∼48 d after the detection. These data allow us to perform a comprehensive study of the emission and physical properties of this event. We also compare these properties with GRBs observed at high and low redshift.

Methods. We built the X-ray, near-infrared, and radio light curves and studied their temporal evolution. Moreover, we investigated the spectral energy distribution (SED) at different times to trace possible spectral evolution. We also compared the prompt phase properties, X-ray luminosity, and optical extinction of GRB 240218A with those of the long-duration GRB (LGRB) population.

Results. The SED analysis reveals a typical afterglow-like behaviour at late times. The origin of the early-time emission is uncertain, with the probable presence of an additional contribution on top of the afterglow emission. From the broadband physical modelling of the afterglow, we identify a narrow Gaussian jet seen slightly off-axis, θ_v = 2.52_−0.29^+0.57 deg, and pinpoint the presence of a possible jet break ∼0.86 d after the trigger.

Conclusions. The results of the analysis and the comparison with other high-z GRBs reveal that we can consider GRB 240218A as a ‘standard’ high-redshift LGRB: the prompt phase properties, the X-ray luminosity, and the optical extinction are consistent with the values derived for the LGRB population. The jet opening angle is narrower but compatible with those of high-z bursts, possibly pointing to more collimated jets at high redshift.