Discovery and analysis of afterglows from poorly localized GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

ABSTRACT Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose challenges for optical follow-up because of their large initial localization regions, leaving many GRBs without identified afterglows. The Gravitational-wave Optical Transient Observer (GOTO), with its wide field of view, dual-site coverage, and robotic rapid-response capability, bridges this gap by rapidly identifying and localizing afterglows from alerts issued by space-based facilities including Fermi, SVOM, Swift, and the EP, providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localization and multiband follow-up of five Fermi/GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two MAXI/GSC (240122A and 240225B) triggered long GRBs discovered by GOTO in 2024. Spectroscopy for six GRBs (no spectroscopy for GRB 241002B) with VLT/X-shooter and GTC/OSIRIS yields precise redshifts spanning $z\approx 0.40$–3.16 and absorption-line diagnostics of hosts and intervening systems. Radio detections for four events (240122A, 240619A, 240910A, and 240916A) confirm the presence of long-lived synchrotron emission. Prompt-emission analysis with Fermi and MAXI data reveals a spectrally hard population, with two bursts lying $>3\sigma$ above the Amati relation. Although their optical afterglows resemble those of typical long GRBs, the prompt spectra are consistently harder than the long-GRB average. Broad-band afterglow modelling of six GOTO-discovered GRBs yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies $E_{\rm jet}\sim 10^{51}$–$10^{52}$ erg, consistent with the canonical long-GRB population. These findings suggest that optical discovery of poorly localized GRBs is likely subject to observational biases favouring luminous events with high spectral peak energy ($E_{\rm p}$), while also providing insight into jet microphysics and central engine diversity.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-10-02
DOI
https://doi.org/10.1093/mnras/staf1689
Primary Topic
Gamma-ray bursts and supernovae
Type
article
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article

Discovery and analysis of afterglows from poorly localized GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

K. Wiersema, J. F. Agüí Fernández, D O’Neill, J. D. Lyman et al.
Monthly Notices of the Royal Astronomical Society
Gamma-ray bursts and supernovae
article

Discovery and analysis of afterglows from poorly localized GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

K. Wiersema, J. F. Agüí Fernández, D O’Neill, J. D. Lyman, Jacob Wise, Raya Dastidar, S. Giarratana, J. T. Palmerio, K. Noysena, Rob A. J. Eyles-Ferris, N. Tanvir, G. E. Anderson, Shouqun Jiang, Benjamin Schneider, S. Belkin, G. Pugliese, B. Sbarufatti, A. de Ugarte Postigo, D. Pollacco, A. J. Levan, A. Martín-Carrillo, J. P. U. Fynbo, R. P. Breton, Y Fang, Danny T. H. Steeghs, K. Ackley, Benjamin P. Gompertz, Libo He, Mark R Kennedy, D. Malesani, Brajesh Kumar, E. Maiorano, Gavin Ramsay, Laura Cotter, A. J. van der Horst, M. De Pasquale, S. Y. Fu, Y. D. Hu, Thomas L. Killestein, Dawei Xu, S. Moran, A. Rossi, Justyn R. Maund, A. Pozanenko, R. Kotak, R. A. M. J. Wijers, M. Á. Aloy, D. H. Hartmann, Amit Kumar, D. L. Coppejans, D. A. Perley, E. Klunko, Luca Izzo, D. K. Galloway, Bob Warwick, A. Saccardi, K. Ulaczyk, Jorge Casares, Christopher A. Phillips, A. J. Castro‐Tirado, V. S. Dhillon, Jiabin An, C. C. Thöne, Martin J. Dyer, Y. Kawakubo, R. L. C. Starling, A. Volnova, D. L. A. Pieterse, Amitabh Gulati, B. Godson, S. Bijavara Seshashayana, Nusrin Habeeb, Y Julakanti, B. C. Rayson, I. Worssam, Xiaowei Liu, Xiang Liu, S. D. Vergani, X. Chen, G. Lombardi, P O’Brien, Z. Zhu, I. V. Allen, M. Serino, V. D’Elia, N. Pankov, Aleksandra Bochenek, Makenzie E. Wortley
article en

Abstract

ABSTRACT Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose challenges for optical follow-up because of their large initial localization regions, leaving many GRBs without identified afterglows. The Gravitational-wave Optical Transient Observer (GOTO), with its wide field of view, dual-site coverage, and robotic rapid-response capability, bridges this gap by rapidly identifying and localizing afterglows from alerts issued by space-based facilities including Fermi, SVOM, Swift, and the EP, providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localization and multiband follow-up of five Fermi/GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two MAXI/GSC (240122A and 240225B) triggered long GRBs discovered by GOTO in 2024. Spectroscopy for six GRBs (no spectroscopy for GRB 241002B) with VLT/X-shooter and GTC/OSIRIS yields precise redshifts spanning $z\approx 0.40$–3.16 and absorption-line diagnostics of hosts and intervening systems. Radio detections for four events (240122A, 240619A, 240910A, and 240916A) confirm the presence of long-lived synchrotron emission. Prompt-emission analysis with Fermi and MAXI data reveals a spectrally hard population, with two bursts lying $>3\sigma$ above the Amati relation. Although their optical afterglows resemble those of typical long GRBs, the prompt spectra are consistently harder than the long-GRB average. Broad-band afterglow modelling of six GOTO-discovered GRBs yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies $E_{\rm jet}\sim 10^{51}$–$10^{52}$ erg, consistent with the canonical long-GRB population. These findings suggest that optical discovery of poorly localized GRBs is likely subject to observational biases favouring luminous events with high spectral peak energy ($E_{\rm p}$), while also providing insight into jet microphysics and central engine diversity.

Monthly Notices of the Royal Astronomical SocietyVol. 544(2)
University College Dublin (IE), University of Messina (IT), National Research University Higher School of Economics (RU), University of Copenhagen (DK), Commonwealth Scientific and Industrial Research Organisation (AU), Centre National de la Recherche Scientifique (FR), The University of Sydney (AU), Aoyama Gakuin University (JP), University of Hertfordshire (GB), Radboud University Nijmegen (NL), Guangxi University (CN), University of Leicester (GB), Moscow Institute of Physics and Technology (RU), University of Turku (FI), Universidad de La Laguna (ES), Universitat de València (ES), Malmö University (SE), Royal Holloway University of London (GB), Yunnan University (CN), George Washington University (US), Chinese Academy of Sciences (CN), Université Paris Cité (FR), Curtin University (AU), Aix-Marseille Université (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris Sciences et Lettres (FR), University College Cork (IE), Université Paris-Saclay (FR), Armagh Observatory & Planetarium (GB), Observatoire de Paris (FR), Agenzia Spaziale Italiana (IT), Space Research Institute (RU), Parc Científic de la Universitat de València (ES), Astronomical Observatory of Capodimonte (IT), Brera Astronomical Observatory (IT), Evgeni Kharadze Georgian National Astrophysical Observatory (GE), University of Manchester (GB), University of Warwick (GB), Sorbonne Université (FR), Dawn Research (United States) (US), Calar Alto Observatory (ES), Astrophysique, Instrumentation et Modélisation (FR), Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna (IT), Millennium Institute of Astrophysics (CL), Institute of Solar-Terrestrial Physics (RU), Institute for Globally Distributed Open Research and Education (SE), Galaxies, Etoiles, Physique et Instrumentation (FR), DAWN Center for Independent Living (US), Physical Sciences (United States) (US), Nordic Optical Telescope (ES), ARC Centre of Excellence for Gravitational Wave Discovery (AU), CEA Paris-Saclay (FR), Instituto de Astrofísica de Canarias (ES), Astronomical Institute of the Slovak Academy of Sciences (SK), Gran Telescopio Canarias (Spain) (ES), Astronomy and Space (AU), Instituto de Astrofísica de Andalucía (ES), International Centre for Radio Astronomy Research (AU), National Astronomical Observatories (CN), Huazhong University of Science and Technology (CN), Monash University (AU), University of Portsmouth (GB), Liverpool John Moores University (GB), University of Birmingham (GB), Clemson University (US), Universidad de Málaga (ES), National Astronomical Research Institute of Thailand (TH), National Institute for Astrophysics (IT), University of Amsterdam (NL), University of Sheffield (GB)
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Gamma-ray bursts and supernovae
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