TITLE:   GCN CIRCULAR
NUMBER:  44660
SUBJECT: GRB 260516D: GOTO optical counterpart detection
DATE:    26/05/17 08:55:51 GMT
FROM:    Sergey Belkin at Monash University <sergey.belkin@monash.edu>

S. Belkin, B. P. Gompertz, M. Wortley, G. Ramsay, D. O'Neill, R. Starling, 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. 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 SVOM/ECLAIRs detection of the possible short faint burst GRB 260516D (Brunet et al., GCN 44649).

Observations of the field were obtained with GOTO-N on 2026-05-16 and 2026-05-17 UT in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline (Lyman et al. 2026). Difference imaging was performed using deeper template observations of the same pointings.

We detect the optical afterglow candidate reported by SVOM/VT (Li et al., GCN 44654), the GROWTH-India Telescope (Mohan et al., GCN 44655), GTC/OSIRIS+ (Izzo et al., GCN 44656), SVOM/COLIBRÍ (García García et al., GCN 44657), and LCO (Wu et al., GCN 44659). The source was initially detected with L = 19.29 +/- 0.05 AB mag at 2026-05-16 22:03:56 UT (T0 +1.40 hr), before fading to L = 21.01 +/- 0.34 AB mag at 2026-05-17 01:26:52 UT (T0 +4.78 hr). These two detections imply a preliminary temporal decay index of alpha_L = 1.30 +/- 0.26, consistent with the optical decay reported by García García et al. (GCN 44657).

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).