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

Kumar, A, Gompertz, BP, Schneider, B, Belkin, S, Wortley, ME, Saccardi, A, O'Neill, D, Ackley, K, Rayson, B, Postigo, ADU, Gulati, A, Steeghs, D, Malesani, DB, Maund, JR, Dyer, MJ, Giarratana, S, Serino, M, Julakanti, Y, Kumar, B, Xu, D et al (2025) Discovery and analysis of afterglows from poorly localized GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey. Monthly Notices of the Royal Astronomical Society, 544 (2). pp. 1541-1587. ISSN 0035-8711

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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 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 Fermi , SVOM , Swift , and the EP , providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localization and multiband follow-up of five Fermi /GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two MAXI /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 z ≈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 Fermi and MAXI data reveals a spectrally hard population, with two bursts lying > 3 σ 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 Ejet ∼ 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 ( Ep ), while also providing insight into jet microphysics and central engine diversity.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	26 September 2025
Date of first compliant Open Access:	27 February 2026
Date Deposited:	27 Feb 2026 14:12
Last Modified:	27 Feb 2026 14:12
DOI or ID number:	10.1093/mnras/staf1689
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28158

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