TL;DR: The GOTO collaboration reported two tentative optical candidates for GRB 260910B after covering about 87.3% of its sky probability, and asked for follow-up to test any link to the burst.

Two tentative candidates reported for GRB 260910B

The Gravitational-wave Optical Transient Observer (GOTO) collaboration reported optical observations in response to GRB 260910B, attributed in the report to the Fermi GBM Team, and identified 2 tentative candidate optical counterparts within the 90% probability contour, according to GCN Circular 45552.

The report was submitted by B. P. Gompertz, S. Belkin, R. Starling, B. Godson, G. Ramsay, D. O'Neill, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, T. Killestein, A. Kumar and M. Pursiainen on behalf of the GOTO collaboration, and sent from Ben Gompertz at the University of Birmingham.

The candidates are described as tentative, not confirmed afterglows. The collaboration explicitly encouraged follow-up observations to determine the nature of these sources and any relationship with GRB 260910B.

No evidence of these sources prior to the GRB trigger time was found in previous GOTO observations, in ZTF observations provided by the Lasair broker, or in the ATLAS forced photometry server, according to the same circular. That archival check is central to why the two sources were highlighted rather than dismissed as known variables.

Why wide-area optical coverage matters for this burst

Gamma-ray bursts can occur anywhere in the sky, are often poorly localized, and fade very quickly, which implies very rapid slewing and a wide field of view, according to the Livermore Optical Transient Imaging System history of robotic burst follow-up.

That history notes that the Livermore system was fully automated and connected via Internet socket to the Gamma-ray Burst Coordinates Network, which analyzes telemetry and delivers coordinate information in real time. The same account describes optics built from four commercial telephoto lenses with custom cameras covering a 17.6 x 17.6 degree field, later upgraded with cooled cameras.

The relevance for GRB 260910B is the size of the search area. GOTO reported covering 396.4 square degrees within the 90% localisation contour, or about 87.3% of the total 2D localisation probability. Wide-field tiling, rather than a single pointed exposure, was therefore required to address the Fermi/GBM localisation region.

The later Super-LOTIS history also illustrates the trade between field size and depth: the original wide-field approach could not see faint sources, while a 0.6-meter telescope gave deeper imaging over a smaller field, first 51 by 51 arcminutes and later 17 by 17 arcminutes at secondary focus. GOTO's reported average 5-sigma depth of 21.0 mag shows the depth side of that trade for the GRB 260910B search.

Observation window, exposures and sky coverage

Observations covering the localisation area began at 2026-09-11 09:02:43 UT, reported as +9.77 hours post trigger, and continued through to 2026-09-11 10:30:32 UT, reported as +11.23 hours post trigger, according to GCN Circular 45552.

The observing sequence consisted of 4x90-second exposures in the GOTO L-band, specified as 400-700 nm. A total of 131 images were taken across 10 unique pointings.

Within the 90% localisation contour, the imaged area totaled 396.4 square degrees. The collaboration estimated coverage at about 87.3% of the total 2D localisation probability, with an average 5-sigma depth of 21.0 mag.

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on candidates passing those checks.

The cited methods are identified in the circular as the GOTO pipeline of Lyman et al. 2026, the classifier of Killestein et al. 2021, ZTF observations provided by the Lasair broker of Smith et al. 2019, ATLAS forced photometry server of Shingles et al. 2021, and calibration with ATLAS-REFCAT2 of Tonry et al. 2018.

Candidate GOTO26jjg: two-epoch detection on the 13% contour

One candidate is listed under internal name GOTO26jjg. Its discovery image and finding chart were made available through the GOTO Marshall page for GOTO26jjg, according to GCN Circular 45552.

The circular states that GOTO26jjg is spatially coincident with a galaxy on the 13% contour of the Fermi/GBM localisation region. It was detected in two epochs, at L = 20.58 ± 0.19 at 09:02:46 UTC on 2026-09-11, or t0 + 9.77 hours, and at L = 20.68 ± 0.23 at 10:10:06 UTC on 2026-09-11, or t0 + 10.90 hours.

The collaboration reported no evidence for fading in this candidate, while noting that the observations are consistent with fading being possible. The two measurements overlap within their stated uncertainties, so the report does not claim a declining light curve.

The most recent GOTO non-detection at that position was L > 20.75 at 3-sigma at 09:54:30 on 2026-09-01, reported as t0 minus 9.56 days. No prior counterpart was found in GOTO, ZTF via Lasair, or ATLAS forced photometry.

Magnitudes were calibrated using ATLAS-REFCAT2 and not corrected for Galactic extinction. That calibration note applies to the reported photometry and affects comparison with measurements in other filters or catalogs.

Candidate GOTO26jjj: single-epoch detection on the 89% contour

The second candidate is listed under internal name GOTO26jjj, with J2000 coordinates RA 264.309139 and Dec 10.532956 and discovery magnitude 20.28 ± 0.18 AB, according to GCN Circular 45552.

The table in the circular leaves the IAU name field blank for this entry. The source is described as spatially coincident with a galaxy on the 89% probability contour of the Fermi/GBM localisation region.

It was covered only in the single epoch reported, beginning at 09:30:34 UTC on 2026-09-11, or t0 + 10.22 hours. Unlike GOTO26jjg, therefore, it has no second GOTO epoch in this dataset to test stability or fading.

The most recent GOTO non-detection was L > 20.61 at 3-sigma at 21:17:42 UTC on 2026-08-31, reported as t0 minus 10.08 days. As with the other candidate, archival checks found no evidence of the source before the trigger in GOTO, ZTF through Lasair, or ATLAS forced photometry.

The circular again notes calibration with ATLAS-REFCAT2 without correction for Galactic extinction, and invites follow-up to establish the nature of the source and any connection to GRB 260910B.

How the candidates were selected and vetted

The search used immediate processing after acquisition, difference imaging against deeper templates, automated filtering, catalog cross-matching, and real-time human vetting, according to GCN Circular 45552.

The classifier step is attributed to Killestein et al. 2021. Candidates were cross-matched against contextual and minor planet catalogs, a step intended to reduce contamination from known objects and solar-system interlopers.

Human vetting was carried out in real time on candidates passing the automated checks. Only two sources survived as tentative candidate optical counterparts inside the 90% probability contour.

The broader robotic-transient context helps explain that sequence. The Livermore Optical Transient Imaging System account notes routine operation beginning in October 1996, with limiting magnitude near 11.5, later about 14 after cooled-camera upgrades, and operation until at least 2001 without successfully detecting an optical counterpart, though it set upper limits.

That earlier program also used idle time between triggers to survey the sky nightly for new transients. GOTO's use of deeper templates, difference imaging, Lasair-provided ZTF data, and ATLAS forced photometry represents the newer version of the same need: separate genuinely new sources from the large background of known or moving objects.

Telescopes, sites and collaboration behind the search

GOTO is described as a network of telescopes principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in New South Wales, Australia, according to GCN Circular 45552.

The consortium listed in the circular includes the University of Warwick, Monash University, Armagh Observatory and Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand, the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias.

The full project name is given as the Gravitational-wave Optical Transient Observer, with references to Steeghs et al. 2022 and Dyer et al. 2024. The report itself was issued on behalf of that collaboration rather than as a single-observatory result.

For U.S. readers, the practical point is coordination: a Fermi burst localisation covering hundreds of square degrees cannot be addressed efficiently by one narrow-field telescope alone. The reported 10 pointings, 131 images, and 87.3% probability coverage show a network attempting to tile most of the uncertainty region to a depth near 21st magnitude within about 10 to 11 hours after the trigger.

What remains unproven and what observers are asked to do

The two sources remain candidates. The circular does not claim either is the afterglow, does not report spectroscopy, and does not report X-ray or radio confirmation, according to GCN Circular 45552.

For GOTO26jjg, the lack of demonstrated fading keeps the interpretation open. For GOTO26jjj, single-epoch coverage likewise prevents a variability test from this dataset alone.

The collaboration's request is explicit: follow-up observations to determine the nature of these sources and any relationship with GRB 260910B are encouraged. Additional photometry can test fading, color, and host-galaxy association, while spectroscopy can test redshift and transient type.

Until such data are reported, the verified result is limited to the search parameters and the two galaxy-coincident transients with no pre-trigger counterpart in the checked archives. That is the record other observatories must use when deciding whether to commit larger apertures.