A1 Refereed original research article in a scientific journal
GOTO identification and broad-band modelling of the counterpart to the SVOM GRB 250818B; 
Authors: Belkin, S; Lamb, G. P.; Ackley, K.; Wortley, M. E.; McGee, S.; Schroeder, G.; Shrestha, M.; Gompertz, B. P.; Galloway, D. K.; Starling, R.; Fong, W.-f.; Laskar, T.; Liu, C.; Gordon, A. C.; Pankov, N.; Volvach, A. E.; Volvach, L. N.; Shein, A.; Pozanenko, A.; Dyer, M. J.; Lyman, J.; Ulaczyk, K.; Steeghs, D.; Dhillon, V. S.; O’Brien, P.; Ramsay, G.; Noysena, K.; Kotak, R.; Breton, R. P.; Nuttall, L. K.; Pollacco, D.; Awiphan, S.; Casares, J.; Chote, P.; Chrimes, A.; Eyles-Ferris, R.; Godson, B.; Irawati, P.; Jarvis, D.; Julakanti, Y.; Kelsey, L.; Kennedy, M. R.; Killestein, T.; Kumar, A.; Levan, A.; Littlefair, S.; Magee, M.; Mandhai, S.; Mata Sánchez, D.; Mattila, S.; McCormac, J.; Mkrtichian, D.; Moran, S.; Mullaney, J.; O’Neill, D.; Patel, M.; Pu, K.; Pursiainen, M.; Sahu, A.; Sawangwit, U.; Stanway, E.; Sun, Y.; Warwick, B.; Wiersema, K.
Publisher: Oxford University Press (OUP)
Publication year: 2026
Journal: Monthly Notices of the Royal Astronomical Society
Article number: stag1193
Volume: 550
Issue: 2
ISSN: 0035-8711
eISSN: 1365-2966
DOI: https://doi.org/10.1093/mnras/stag1193
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1093/mnras/stag1193
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/527009641
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
We report the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localized by the Gravitational-wave Optical Transient Observer (GOTO). We compile X‑ray, optical/near‑infrared and radio measurements into broadband light curves and spectral energy distributions. Although the burst was initially reported as short, the public prompt light curve indicates a duration of ≈5 s, which argues against a secure short‑duration classification. The afterglow is unusually bright for short‑GRB samples: in X‑rays and optical it is broadly consistent with the long‑GRB population, while in the radio it ranks among the most luminous events in the short‑GRB comparison sample and lies within the long‑GRB range. MeerKAT detects the source at 3.1 GHz, and deep higher‑frequency limits are provided by the Atacama Large Millimetre/submillimetre Array (ALMA). Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving z = 1.216. Synchrotron forward‑shock modelling favours a constant‑density medium with refreshed (energy‑injection) emission, and is well described by a two‑component jet with core isotropic‑equivalent kinetic energy E0 ≃ 8 × 1052 erg, ambient number density n0 ≃ 12 cm−3, jet half‑opening angle θj ≃ 0.13 rad (≈7.4°), and electron index p ≃ 1.67. We do not identify a host conclusively: the data permit either a faint LS DR10 galaxy at ≈4 arcsec (≈34 kpc) as an offset candidate, or a fainter, near‑coincident uncatalogued host. GRB 250818B illustrates the power of rapid counterpart identification in the SVOM era, while the uncertain prompt classification and inconclusive host association still limit definitive progenitor interpretation.
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The GOTO project acknowledges the support of the Monash-Warwick Alliance; University of Warwick; Monash University; University of Sheffield; University of Leicester; Armagh Observatory & Planetarium; the National Astronomical Research Institute of Thailand (NARIT); Instituto de Astrofísica de Canarias (IAC); University of Portsmouth; University of Turku; University of Birmingham; and the UK Science and Technology Facilities Council (STFC; grant numbers ST/T007184/1, ST/T003103/1, and ST/Z000165/1). The National Radio Astronomy Observatory and Green Bank Observatory are facilities of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2024.1.01131.T. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSTC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center online service, provided by the NASA/Goddard Space Flight Center. W. M. Keck Observatory access was supported by Northwestern University and theCenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. GPL acknowledges support from the Royal Society (grant nos. DHF-R1-221175 and DHF-ERE-221005). ACG and the Fong Group at Northwestern acknowledge support by the National Science Foundation under grant nos. AST-1909358, AST-2206494, AST-2308182, and CAREER grant no. AST-2047919. W. M. Keck Observatory access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). MEW is supported by theScience and Technology Facilities Council (STFC). BPG acknowledges support from STFC grant no. ST/Y002253/1. BPG and DO acknowledge support from the Leverhulme Trust grant no. RPG-2024-117. RLCS and SM are supported by Leverhulme Trust grant RPG-2023-240. YS is supported by the Chinese Scholarship Counciland the University of Leicester. DMS acknowledges support through the Ramón y Cajalgrant RYC2023-044941, funded by MCIU/AEI/10.13039/501100011033 and FSE + . TLK acknowledges support from a Warwick Astrophysics prize post-doctoral fellowship made possible thanks to a generous philanthropic donation. SM acknowledges financial support from the Research Council of Finland project 350458. The authors express their gratitude to the Terskol Observatory Collective Use Center for organizing and making it possible to conduct observations using the Zeiss-2000 telescope of the INASAN observatory.
