GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B
S Belkin, G P Lamb, K Ackley, M E Wortley, S McGee, G Schroeder, M Shrestha, B P Gompertz, D K Galloway, R Starling, W-f Fong, T Laskar, C Liu, A C Gordon, N Pankov, A E Volvach, L N Volvach, A Shein, A Pozanenko, M J Dyer, J Lyman, K Ulaczyk, D Steeghs, V S Dhillon, P O’Brien, G Ramsay, K Noysena, R Kotak, R P Breton, L K Nuttall, D Pollacco, S Awiphan, J Casares, P Chote, A Chrimes, R Eyles-Ferris, B Godson, P Irawati, D Jarvis, Y Julakanti, L Kelsey, M R Kennedy, T Killestein, A Kumar, A Levan, S Littlefair, M Magee, S Mandhai, D Mata Sánchez, S Mattila, J McCormac, D Mkrtichian, S Moran, J Mullaney, D O’Neill, M Patel, K Pu, M Pursiainen, A Sahu, U Sawangwit, E Stanway, Y Sun, B Warwick, K WiersemaAbstract
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.