Early Multiwavelength Observations of AT 2026fgk: The Luminous Afterglow to Sub-luminous GRB 260310A, Identified Independently of a Gamma-ray Trigger

The origins of sub-luminous (Lγ,iso < 1049.5\,erg\,s⁻¹) gamma-ray bursts (GRBs) associated with broad-lined Type~Ic supernovae (Ic-BL SNe) are poorly understood, in part due to the low discovery rate and faint afterglows. Here we present the identification of the optical afterglow of Fermi-GBM-detected GRB\,260310A (AT\,2026fgk) as a rapidly rising ($>1\,$mag\,d⁻¹), red ($g-r=0.4$\,mag) transient using the Gravitational-wave Optical Transient Observatory, Large Array Survey Telescope, and Zwicky Transient Facility (ZTF) data streams. We present multiwavelength follow-up observations from the first 50\,days, which reveal that GRB 260310A/AT\,2026fgk was sub-luminous (L_γ,iso=1048.8\,erg\,s⁻¹); it was the most nearby ($z=0.153$) afterglow identified blindly by an optical survey; and that it is one of the brightest afterglows ever observed at X-ray, optical, and radio (cm to mm) wavelengths. We spectroscopically confirm an underlying Ic-BL SN with properties typical of GRB-SNe (Mₑj≈3\,M_, EK≈ 10⁵²\,erg). With basic modeling of the afterglow, including the long optical rise (≈10³\,s), we infer either a low initial Lorentz factor (Γ₀≈40) or a slightly off-axis viewing angle (3^∘). The host galaxy's mass and star formation rate are similar to the hosts of other sub-luminous GRBs. ZTF's flux-limited survey gives a volumetric rate of AT\,2026fgk-like events of 0.30+1.37-0.29\,Gpc\,⁻³\,yr⁻¹, which is consistent with the on-axis, high luminosity (Lγ,iso>1049.5\,erg\,s⁻¹) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low-Γ₀ bursts and the beaming of the initial relativistic material in GRBs.