GCN Circular 44265

D. O'Neill, R. Starling, G. Ramsay, 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, B. Gompertz, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, on behalf of GOTO collaboration

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the  Fermi/GBM alert GRB 260411B (Fermi GBM team, GCN #44262
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 ).

Observations covering the localisation area began at 2026-04-11 08:57:53 UT (t0 +0.89h). 8 images consisting of 4 x 90s exposures were taken in the L band (400-700 nm), covering 38.5 sq deg within the 90% localisation contour. ~32.3% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.4 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 (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.

A new optical source GOTO26dam is identified within the GBM 90% localisation region (22% contour) with co-ordinates:

    RA,DEC (J2000) = 85.567893, 1.984983,
                    05:42:16.29, +01:59:05.94

The source was detected with L = 17.55 ± 0.02 AB mag (+0.89h) and is rapidly fading with a decay index alpha ~ -2.1±0.4. We find no evidence of the source prior to the GRB trigger time in previous ATLAS observations at 2026-04-07 17:36:17.597 (t0 -86.47h) down to c > 19.37 mag using the forced photometry server (Shingles et al. 2021).

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the  University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).