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Abstract:

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 $\sim 5$ 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 $E_{0}\sim 8\times 10^{52}$ erg, ambient number density $n_0\sim 12\, \mathrm{cm^{-3}}$, jet half-opening angle $\theta _j\simeq 0.13$ rad ($\sim 7.4^\circ$), and electron index $p\simeq 1.67$. We do not identify a host conclusively: the data admit either a faint LS DR10 galaxy at $\sim 4\, {\rm arcsec}$ ($\sim 34$ 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.

Abstract:

We present constraints on the amplitude of matter fluctuations from the clustering of galaxies and their cross-correlation with the gravitational lensing convergence of the cosmic microwave background (CMB), focusing on low redshifts (z ≲ 0.3), where potential deviations from a perfect cosmological constant dominating the growth of structure could be more prominent. Specifically, we make use of data from the 2MASS photometric survey (2MPZ) and the WISE×SuperCOSMOS galaxy survey, in combination with CMB lensing data from Planck. Using a hybrid effective field theory (HEFT) approach to model galaxy bias we obtain constraints on the combination S 8 = σ 8 √(Ω m /0.3), where σ 8 is the amplitude of matter fluctuations, and Ω m is the non-relativistic matter fraction. Using a prior on Ω m based on the baryon acoustic oscillation measurements of DESI, we find S 8 = 0.79 ± 0.06, in reasonable agreement with CMB constraints. We also find that, in the absence of this prior, the data favours a value of Ω m = 0.245 ± 0.024, that is 2.8σ lower than Planck. This result is driven by the broadband shape of the galaxy auto-correlation, and may be affected by theoretical uncertainties in the HEFT power spectrum templates. We further reconstruct the low-redshift growth history, finding it to be compatible with the Planck predictions, as well as existing constraints from lensing tomography. Finally, we study our constraints on the HEFT bias parameters of the galaxy samples studied, finding them to be in reasonable agreement with coevolution predictions.