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

TL;DR

This work demonstrates that the Gravitational-wave Optical Transient Observer (GOTO) can rapidly identify optical afterglows for poorly localised GRBs, bridging large gamma-ray error regions to precise optical positions suitable for multi-wavelength follow-up. By surveying seven long GRBs in 2024 (two MAXI-triggered serendipitous events and five Fermi-triggered responses), GOTO enabled early optical detections and coordinated X-ray, UV/optical/NIR, and radio campaigns that yielded redshifts from VLT/X-shooter and GTC/OSIRIS for six events. Prompt-emission analyses reveal unusually hard spectra and, in some cases, outliers to the Amati relation, while afterglow modelling with a TopHat jet framework yields beaming-corrected jet energies around 10^51–10^52 erg and jet core angles of a few degrees, placing these bursts within the canonical LGRB population. The results highlight observational biases that favour luminous, hard-spectrum GRBs in optical searches and underscore GOTO’s utility for rapid, wide-field discovery, enabling robust, multi-messenger astrophysics with broad redshift coverage. Collectively, the study establishes GOTO as a scalable, high-impact facility for identifying and characterising optical counterparts to poorly localised GRBs, with implications for jet physics, progenitor diversity, and future multi-messenger campaigns.

Abstract

Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose a challenge for optical follow-up due to 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 EP, providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localisation and multiband follow-up of five Fermi/GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two MAXI/GSC (240122A and 240225B) triggered long GRBs (LGRBs) discovered by GOTO in 2024. Spectroscopy for six GRBs (no spectroscopic data for GRB 241002B) with VLT/X-shooter and GTC/OSIRIS yields precise redshifts spanning $z\approx0.40-$3.16 and absorption-line diagnostics of host 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 LGRBs, the prompt spectra are consistently harder than the LGRBs' average. Consistent modelling of six GOTO-discovered GRB afterglows yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies ($E_{jet}\sim10^{51-52}$)erg, consistent with the canonical LGRB population. These findings suggest that optical discovery of poorly localised GRBs may be subject to observational biases favouring luminous events with high spectral peak energy, while also providing insight into jet microphysics and central engine diversity.

Figures (27)

• Figure 1: The full configuration of the GOTO telescope network in April 2023, comprising 32 robotic unit telescopes distributed across four domes, two domes at each of the two sites. Top: GOTO-N, located at the Observatorio del Roque de los Muchachos on La Palma, comprising GOTO-1 (left) and GOTO-2 (right). Bottom: GOTO-S, hosted at Siding Spring Observatory in Australia, consisting of GOTO-3 (left) and GOTO-4 (right). Figure credit: Dyer2024SPIE.
• Figure 2: A summary of the GOTO GRB follow-up strategy.
• Figure 3: GOTO coverage of each of the GRBs in the sample. The first two plots denote the 90% containment MAXI/GSC localisations (red) in a $3\degree\times3\degree$ field. The localisation areas are generated based on information from their discovery GCNs. The following five plots show Fermi/GBM localisations (grey) and the 1 and 2 $\sigma$ contours from their respective HEALPix skymaps in a $20\degree\times20\degree$ field. In all plots, the 2D footprint of GOTO images taken in the first 10 hr post-trigger that overlap the localisations are shown in light blue. The corresponding afterglow positions are marked with a cyan star.
• Figure 4: Finding charts of GRBs 240122A, 240225B, 240619A, 240910A, 240916A, 241002B, and 241228B in the GOTO $L-$band (400--700 nm) observed by GOTO. Each cutout is a $300 \times 300$ pixel region centred on the transient, corresponding to a FoV of $\sim6.3\arcmin \times 6.3\arcmin$ at the GOTO pixel scale of $1.26\arcsec$/pix. For comparison, survey images from the Legacy Survey DR10 are shown (except for GRB 240122A, where a Pan-STARRS DR1 image is used), matched to the same FoV. Details of each object are listed in Table \ref{['tab:sample']}.
• Figure 5: First detection times across various observatories for GRBs in our sample. Shown are prompt (red), X-ray afterglow (blue), UV/optical/NIR afterglow (green), and radio afterglow (purple) observations from both space- and ground-based facilities. In all cases, GOTO discovered the optical afterglow following the prompt emission. For GRB 240619A, although ATLAS has the earliest epoch, the afterglow was first discovered by GOTO, and the ATLAS data were serendipitously pre-covered and used in the GOTO discovery report Gompertz2024GCN36715.