Kumar, A., Gompertz, B.P., Schneider, B. et al. (86 more authors) (2025) Discovery and analysis of afterglows from poorly localised GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey. Monthly Notices of the Royal Astronomical Society. ISSN: 0035-8711
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 localisation 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 localising afterglows from alerts issued by space-based facilities including Fermi, SVOM, Swift and the EP, providing early optical positions for coordinated multi-wavelength follow-up. In this paper, we present optical afterglow localisation and multi-band follow-up of seven Fermi/GBM and MAXI/GSC triggered long GRBs (240122A, 240225B, 240619A, 240910A, 240916A, 241002B, and 241228B) 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 ≈ 0.40–3.16 and absorption-line diagnostics of hosts and intervening systems. Radio detections for four events 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 >3σ 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. Broadband afterglow modelling of six GOTO-discovered GRBs yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies Ejet ∼ 1051–1052 erg, consistent with the canonical long-GRB population. These findings suggest that optical discovery of poorly localised GRBs is likely subject to observational biases favouring luminous events with high spectral peak energy (Ep), while also providing insight into jet microphysics and central engine diversity.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © The Author(s) 2025. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. |
Keywords: | transients: gamma-ray bursts; gamma-ray bursts: general; gamma-ray burst: individual: GRB 240122A; GRB 240225B; GRB 240619A; GRB 240910A; GRB 240916A; GRB 241002B; GRB 241228B; techniques: photometric; techniques: spectroscopic |
Dates: |
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Institution: | The University of Sheffield |
Academic Units: | The University of Sheffield > Faculty of Science (Sheffield) > School of Mathematical and Physical Sciences |
Date Deposited: | 08 Oct 2025 13:56 |
Last Modified: | 08 Oct 2025 13:56 |
Status: | Published online |
Publisher: | Oxford University Press (OUP) |
Refereed: | Yes |
Identification Number: | 10.1093/mnras/staf1689 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:232697 |