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

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$^{-3}$, $θ_j \simeq 0.10$ 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.

💡 Research Summary

This paper presents the discovery and comprehensive multi‑wavelength follow‑up of GRB 250818B, a burst detected by the Space Variable Objects Monitor (SVOM) on 2025‑08‑18. The SVOM/ECLAIRs instrument provided a rapid onboard localisation with a 90 % confidence radius of 7.44 arcmin. Within minutes, the Gravitational‑wave Optical Transient Observer (GOTO) began tiling the error region and identified a fading optical counterpart (designated GOTO25fzq/AT2025ukm) at L ≈ 18.7 mag 0.54 h after trigger, which faded to L ≈ 19.5 mag by 1.67 h. Pre‑trigger GOTO images showed no source down to L > 20.3 mag (3σ), confirming the association. Subsequent observations by SVOM/VT, Swift/UVOT, Swift/XRT, Einstein Probe/FXT, and numerous ground‑based facilities (KAIT, NOT, SAO RAS, TRT, Lesedi, LT, GRANDMA/Kilonova‑Catcher, Wendelstein FTW, etc.) provided a dense optical/NIR light curve from minutes to several days.

X‑ray follow‑up with Swift/XRT began 1.7 ks after the trigger, yielding a time‑resolved spectrum (S/N = 10 bins) that showed a decaying flux consistent with the optical position. Radio observations with MeerKAT at 3.1 GHz detected the afterglow in the first three epochs, showing a rise and subsequent decay; a single detection at 1.3 GHz occurred around 15 days. Deep ALMA continuum observations at ∼230 GHz resulted in non‑detections, providing stringent upper limits.

Keck/LRIS spectroscopy of the optical afterglow revealed a continuum with metal absorption lines (Fe II, Mg II, Mg I) establishing a redshift of z = 1.216. This redshift enables rest‑frame modelling of the afterglow and host‑galaxy properties.

The authors compiled all photometry, corrected for Galactic extinction, and constructed broadband spectral energy distributions (SEDs) at multiple epochs. They modelled the afterglow using the afterglowpy framework (via the redback package), testing standard forward‑shock scenarios as well as extensions such as energy injection and multi‑component jets. A single‑component, constant‑density (ISM) model could not simultaneously reproduce the early optical rise and the radio behaviour. The best‑fit solution requires a two‑component jet with refreshed (energy‑injection) emission:

• Isotropic‑equivalent kinetic energy Eₖ,iso ≈ 4 × 10⁵² erg
• Ambient density n₀ ≈ 3.6 cm⁻³ (constant ISM)
• Jet opening angle θⱼ ≈ 0.10 rad (≈ 5.7°)
• Electron power‑law index p ≈ 1.64 (unusually hard)
• A second, narrower jet component injecting ≈10⁵¹ erg at ≈0.5 day, driving the later optical and radio plateau.

These parameters place GRB 250818B among the most luminous short‑GRB afterglows in X‑ray and optical, and among the brightest high‑redshift short‑GRB radio afterglows known.

Host‑galaxy association is ambiguous. The nearest catalogued LS DR10 galaxy (r_AB ≈ 24.7 mag) lies 4 arcsec (≈ 34 kpc at z = 1.216) from the afterglow, with a chance‑alignment probability P_cc ≈ 0.2, insufficient for a secure identification. SED fitting of this candidate (using Prospector) suggests a low‑mass (M_* ≈ 10⁹ M_⊙), low‑star‑formation galaxy, typical of short‑GRB hosts. However, the imaging depth does not rule out a fainter, coincident host below the survey limit, leaving the true host uncertain.

The discussion emphasizes the synergy between SVOM’s rapid, arc‑minute localisations and GOTO’s wide‑field, fast‑response capability, which together enabled sub‑hour identification and dense multi‑wavelength coverage of a high‑z, short‑GRB. The need for energy injection and a structured jet challenges the simple picture of short‑GRB afterglows and suggests more complex outflow dynamics. While host‑association uncertainty hampers offset‑based progenitor arguments (e.g., distinguishing binary neutron‑star mergers from collapsar‑like events), the afterglow data alone provide strong constraints on the circumburst environment and jet physics.

In conclusion, GRB 250818B showcases the power of the SVOM era for rapid counterpart discovery, demonstrates that even nominal short GRBs can exhibit exceptionally bright afterglows requiring refreshed shock models, and highlights that host‑identification remains a limiting factor for fully interpreting progenitor channels. Future deeper imaging and high‑resolution spectroscopy will be essential to resolve the host ambiguity and to refine our understanding of such energetic events.