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

TL;DR

This study presents rapid, multi-wavelength follow-up of GRB 250818B discovered by SVOM and localized optically by GOTO, assembling X-ray to radio data plus spectroscopy to build broadband light curves and SEDs. Forward-shock modelling with energy injection and a two-component jet best explains the unusually bright afterglow, implying high kinetic energies and a constant-density circumburst medium; the inferred redshift $z=1.216$ anchors rest-frame energetics and host properties. A key outcome is the identification of two viable host scenarios, with the offset galaxy being plausible but not definitive, highlighting how host association uncertainties can limit offset-based progenitor inferences. Overall, GRB 250818B demonstrates the power of rapid wide-field counterpart identification for constraining afterglow physics and host environments in the SVOM era, while stressing the ongoing need for deeper imaging and spectroscopy to resolve host associations at high redshift.

Abstract

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.

Figures (20)

• Figure 1: Keck I/LRIS flux-calibrated, Galactic-extinction-corrected spectrum of the optical afterglow of GRB 250818B obtained at $\approx 10.7$ hr post-burst; wavelengths are shown in the observer frame. The light-blue curve shows the original spectrum and the orange curve shows a Gaussian-smoothed version ($\sigma = 2$ Å) for display purposes. Vertical dashed lines mark the expected observed-frame wavelengths of the Fe ii$\lambda\lambda2344, 2374, 2382$, Mg ii$\lambda\lambda2796, 2803$, and Mg i$\lambda2852$ absorption transitions at the best-fit redshift $z = 1.216$. Insets highlight the Fe ii and Mg ii/Mg i regions used to refine the redshift estimate.
• Figure 2: Multi-wavelength light curve of GRB 250818B, showing X-ray, optical/near-infrared, and radio flux densities as a function of time since the SVOM/ECLAIRs trigger. Optical points are corrected for Galactic extinction and converted to AB flux densities. Triangles with arrows indicate $3\sigma$ upper limits. The host-galaxy fluxes shown in $g$, $r$, $i$, and $z$ correspond to the LS DR10 candidate host galaxy (Tractor objid 5790) discussed in Section \ref{['sec:host_association']}. For visual clarity, the plotted $g$-band host level is vertically offset by a factor of $3/4$ to avoid overlap with the $r$-band host level; all analysis uses the unscaled host flux.
• Figure 3: Top: XRT count rate light curve of the three flaring episodes, where the highlighted sections indicate the three SNR = 10 spectral bins. The late-time XRT data at $\sim 3.35$ d does not meet the SNR threshold and so is not included in the spectral analysis. Bottom: evolution of the best-fit photon index with time.
• Figure 4: Swift/XRT observer-frame 0.3--10 keV afterglow comparison for GRB 250818B. Grey curves show 0.3--10 keV XRT light curves of Swift-detected GRBs downloaded from the UKSSDC repository. Short-duration bursts are highlighted in darker grey, while long-duration events are shown in light grey, using a Swift/BAT $T_{90}$-based duration classification from the Swift/BAT GRB summary catalogue (excluding events with unidentified $T_{90}$). Bursts whose BAT $T_{90}\pm\sigma$ interval straddles 2 s ("borderline" between short and long) are plotted in medium grey. Red points show the unabsorbed 0.3--10 keV fluxes of GRB 250818B from our XRT spectral analysis with SNR=10 binning (Table \ref{['tab:xrt_250818B_snr10']}); the red line connects the measurements to guide the eye. Times are measured relative to the prompt trigger in the observer frame (BAT trigger for the Swift comparison sample; SVOM/ECLAIRs $T_{0}$ for GRB 250818B).
• Figure 5: Observer-frame $R$-band afterglow comparison for GRB 250818B. Grey curves show literature $R$-band afterglow light curves compiled from Kann2006Kann2010Kann2011NicuesaGuelbenzu2012. The short/Type I subset (as classified in Kann2011) is highlighted in darker grey, while long/Type II events are shown in light grey. The red curve shows the $R$-band model of GRB 250818B derived from our simultaneous multiband SBPL fit. All magnitudes are corrected for Galactic extinction.