GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B

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2026
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Abstract
Abstract We present the discovery and multi-wavelength follow-up of 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised by the Gravitational-wave Optical Transient Observer (GOTO). We compile X-ray, optical/NIR, and radio data into broadband light curves and SEDs. Despite being initially reported as short, the public prompt light-curve figure suggests a duration of ~5 s, arguing against a secure short-duration classification. The afterglow is unusually bright relative to short-GRB samples: in X-rays and optical it is broadly consistent with the long-GRB population, while in the radio it is among the most luminous events in the short-GRB comparison sample and remains within the long-GRB range. MeerKAT detects the source at 3.1 GHz, while the Atacama Large Millimeter/submillimeter Array (ALMA) provides deep higher-frequency limits. 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 and refreshed (energy-injection) emission, 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 admit either a faint LS DR10 galaxy at ~4″ (~34 kpc) as an offset candidate, or a fainter, near-coincident uncatalogued host. 250818B highlights the power of rapid counterpart identification in the SVOM era, while uncertain prompt classification and inconclusive host association still limit progenitor interpretation.
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Authors S Belkin, Gavin P. Lamb, K Ackley, M E Wortley, S McGee, G Schroeder, M Shrestha, B. P. Gompertz, D K Galloway, R Starling, WF Fong, T Laskar, Chang Liu, Alexa C. Gordon, N Pankov, A E Volvach, L N Volvach, A Shein, A Pozanenko, M. Dyer, J. Lyman, K Ulaczyk, D. Steeghs, V. S. Dhillon, P O’Brien, Gavin Ramsay, K Noysena, R Kotak, R. P. Breton, L K Nuttall, D Pollacco, Supachai Awiphan, J. Casares, P Chote, A A Chrimes, R. A. J. Eyles-Ferris, B Godson, P Irawati, D Jarvis, Y Julakanti, L Kelsey, Mark Kennedy, T. Killestein, A Kumar, A Levan, S Littlefair, M. Magee, S Mandhai, D Mata Sánchez, S Mattila, J McCormac, D. E. Mkrtichian, S Moran, J Mullaney, D O’Neill, M. Patel, K Pu, M. Pursiainen, A Sahu, U Sawangwit, E Stanway, Y Sun, Ben Warwick, K. Wiersema
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1193
URL
https://doi.org/10.1093/mnras/stag1193
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