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

K. -R Hinds, A. Y. Q Ho, Y Wagh, R Jayaraman, D. A Perley, G Waratkar, A Bochenek, B. P Gompertz, C Fremling, J Rastinejad, …
ArXiv.org
03 Jun 2026
DOI: https://doi.org/10.48550/arxiv.2606.05146
url
https://doi.org/10.48550/arXiv.2606.05146View
Preprint (Author's original) Open arXiv.org - Non-exclusive license to distribute

Abstract

Physics - High Energy Astrophysical Phenomena
The origins of sub-luminous ( $L_\mathrm{γ,\mathrm{iso}} < 10^{49.5}$ \,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 ( $L_\mathrm{γ,iso}=10^{48.8}\,$ 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 ( $M_\mathrm{ej}\approx3\,M_\odot$ ,$E_{\rm K}\approx 10^{52}\,$ erg). With basic modeling of the afterglow, including the long optical rise ( $\approx10^{3}\,$ s), we infer either a low initial Lorentz factor ( $Γ_0\approx40$ ) or a slightly off-axis viewing angle ( $\lesssim3^\circ$ ). 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 ( $L_{\rm γ,iso}>10^{49.5}$ \,erg\,s $^{-1}$ ) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low- $Γ_0$bursts and the beaming of the initial relativistic material in GRBs.

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