Discovery and analysis of afterglows from poorly localized GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

Abstract

<p>Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the <em>Fermi</em>/GBM, pose challenges for optical follow-up because of their large initial localization 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 localizing afterglows from alerts issued by space-based facilities including <em>Fermi, SVOM, Swift</em>, and the <em>EP</em>, providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localization and multiband follow-up of five <em>Fermi</em>/GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two <em>MAXI</em>/GSC (240122A and 240225B) triggered long GRBs 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 <i>z</i> ≈ 0.40–3.16 and absorption-line diagnostics of hosts and intervening systems. Radio detections for four events (240122A, 240619A, 240910A, and 240916A) confirm the presence of long-lived synchrotron emission. Prompt-emission analysis with <em>Fermi</em> and <em>MAXI</em> 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. Broad-band afterglow modelling of six GOTO-discovered GRBs yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies <i>E</i>_jet ∼ 10⁵¹–10⁵² erg, consistent with the canonical long-GRB population. These findings suggest that optical discovery of poorly localized GRBs is likely subject to observational biases favouring luminous events with high spectral peak energy (<i>E</i>_p⁠⁠), while also providing insight into jet microphysics and central engine diversity.<br></p>