October 5, 2025Open Access

Discovery and Analysis of Afterglows from Poorly Localised GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

Key Points

Locating afterglows of poorly localised GRBs reveals observational biases in discovering luminous events.
Optical spectroscopy provided redshifts ranging from 0.40 to 3.16 for six GRBs using VLT/X-shooter.
Afterglow modeling indicates jet half-opening angles of a few degrees and kinetic energies around 10^51 to 10^52 erg.
Radio detections confirm long-lived synchrotron emission for four events, enhancing understanding of GRB phenomena.

Abstract

Abstract Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose challenges for optical follow-up because of their large initial localisation regions, leaving many GRBs without identified afterglows. The Gravitational-wave Optical Transient Observer (GOTO), with its wide field of view, dual-site coverage, and robotic rapid-response capability, bridges this gap by rapidly identifying and localising afterglows from alerts issued by space-based facilities including Fermi, SVOM, Swift and the EP, providing early optical positions for coordinated multi-wavelength follow-up. In this paper, we present optical afterglow localisation and multi-band follow-up of seven Fermi/GBM and MAXI/GSC triggered long GRBs (240122A, 240225B, 240619A, 240910A, 240916A, 241002B, and 241228B) discovered by GOTO in 2024. Spectroscopy for six GRBs (no spectroscopy for GRB 241002B) with VLT/X-shooter and GTC/OSIRIS yields precise redshifts spanning z ≈ 0.40–3.16 and absorption-line diagnostics of hosts and intervening systems. Radio detections for four events confirm the presence of long-lived synchrotron emission. Prompt-emission analysis with Fermi and MAXI data reveals a spectrally hard population, with two bursts lying 3σ above the Amati relation. Although their optical afterglows resemble those of typical long GRBs, the prompt spectra are consistently harder than the long-GRB average. Broadband afterglow modelling of six GOTO-discovered GRBs yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies Ejet ∼ 1051–1052 erg, consistent with the canonical long-GRB population. These findings suggest that optical discovery of poorly localised GRBs is likely subject to observational biases favouring luminous events with high spectral peak energy (Ep), while also providing insight into jet microphysics and central engine diversity.

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