GOTO identification and broad-band modelling of the counterpart to the SVOM GRB 250818B

We present the discovery and multiwavelength follow-up of gamma-ray bursts (GRB) 250818B, detected by the Space Variable Objects Monitor ( SVOM ) and localized by the Gravitational-wave Optical Transient Observer. We compile X-ray, optical/near-infrared, and radio data into broad-band light curves and spectral energy distributions. 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 Millimetre/submillimetre Array 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 ∼ 12cm−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 ∼ 4arcsec ( ∼ 34 kpc) as an offset candidate, or a fainter, near-coincident uncatalogued host. GRB 250818B highlights the power of rapid counterpart identification in the SVOM era, while uncertain prompt classification and inconclusive host association still limit progenitor interpretation.