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

Kumar, Amit; Gompertz, B. P.; Schneider, B.; Belkin, S.; Wortley, M. E.; Saccardi, A.; O'Neill, D.; Ackley, K.; Rayson, B.; Postigo, A. de Ugarte; Gulati, A.; Steeghs, D.; Malesani, D. B.; Maund, J. R.; Dyer, M. J.; Giarratana, S.; Serino, M.; Julakanti, Y.; Kumar, B.; Xu, D.; Eyles-Ferris, R. A. J.; Zhu, Z.-P.; Warwick, B.; Hu, Y.-D.; Allen, I.; Ramsay, G.; Starling, R. L. C.; Lyman, J.; Ulaczyk, K.; Godson, B.; Galloway, D. K.; Dhillon, V. S.; O'Brien, P.; Noysena, K.; Kotak, R.; Breton, R. P.; Nuttall, L. K.; Pollacco, D.; Casares, J.; Killestein, T. L.; Kennedy, M. R.; Habeeb, N.; Moran, S.; Wiersema, K.; Worssam, I.; Coppejans, D. L.; Phillips, C. A.; Martin-Carrillo, A.; Pankov, N. S.; Fernández, J. F. Agüí; Aloy, M. A.; An, J.; Anderson, G. E.; Bochenek, A.; Castro-Tirado, A. J.; Chen, X.; Cotter, L.; Dastidar, R.; De Pasquale, M.; D'Elia, V.; Fang, Y.; Fu, S. Y.; Fynbo, J. P. U.; Hartmann, D. H.; He, L. B.; Izzo, L.; Jiang, S. Q.; Kawakubo, Y.; Klunko, E. V.; Levan, A. J.; Liu, X.-W.; Liu, X.; Lombardi, G.; Maiorano, E.; Palmerio, J. T.; Perley, D. A.; Pieterse, D. L. A.; Pozanenko, A. S.; Pugliese, G.; Rossi, A.; Sbarufatti, B.; Seshashayana, S. Bijavara; Tanvir, N. R.; Thöne, C. C.; van der Horst, A. J.; Vergani, S. D.; Volnova, A. A.; Wijers, R. A. M. J.; Wise, J. L.
Bibliographical reference

Monthly Notices of the Royal Astronomical Society

Advertised on:
12
2025
Number of authors
89
IAC number of authors
3
Citations
0
Refereed citations
0
Description
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 $z\approx 0.40$─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 $>3\sigma$ 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 $E_{\rm jet}\sim 10^{51}$─$10^{52}$ 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 ($E_{\rm p}$), while also providing insight into jet microphysics and central engine diversity.
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