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

Belkin, S ORCID: 0000-0002-9798-029X, Lamb, GP ORCID: 0000-0001-5169-4143, Ackley, K, Wortley, ME, McGee, S, Schroeder, G, Shrestha, M, Gompertz, BP ORCID: 0000-0002-5826-0548, Galloway, DK, Starling, R, Fong, W, Laskar, T, Liu, C ORCID: 0000-0002-7866-4531, Gordon, AC ORCID: 0000-0002-5025-4645, Pankov, N, Volvach, AE, Volvach, LN, Shein, A, Pozanenko, A, Dyer, MJ ORCID: 0000-0003-3665-5482 et al (2026) GOTO identification and broad-band modelling of the counterpart to the SVOM GRB 250818B. Monthly Notices of the Royal Astronomical Society, 550 (2). ISSN 0035-8711

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Abstract

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 (Formula presented) 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 (Formula presented). 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 (Formula presented) erg, ambient number density (Formula presented), jet half-opening angle (Formula presented) rad ((Formula presented) ), and electron index (Formula presented). We do not identify a host conclusively: the data admit either a faint LS DR10 galaxy at (Formula presented) ((Formula presented) 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.

Item Type:	Article
Uncontrolled Keywords:	(transients:) gamma-ray bursts; gamma-ray burst: individual: GRB 250818B; 5101 Astronomical Sciences; 51 Physical Sciences; 7 Affordable and Clean Energy; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	19 June 2026
Date of first compliant Open Access:	22 September 2026
Date Deposited:	22 Sep 2026 09:20
Last Modified:	22 Sep 2026 09:20
DOI or ID number:	10.1093/mnras/stag1193
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29498

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