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الملخص الأصلي

كما كتبه المؤلفون، بالإنجليزية arXiv:2602.16559

Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while 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 strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $$E_{K,iso} \sim 4\times10^{52}$$ erg, $$n_0 \sim 3.6$$ cm$$^{-3}$$, $$\theta_j \simeq 0.10$$ rad ($$\sim 5.7$$ deg), and $$p \simeq 1.64$$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($$r_{AB} \sim 24.7$$) is offset by $$\sim 4$$ arcsec ($$\sim 34$$ kpc) with chance-alignment probability $$P_{cc} \sim 0.2$$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.