TITLE: GCN CIRCULAR NUMBER: 23069 SUBJECT: GRB 180728B: GOTO optical search over IPN region DATE: 18/07/31 21:56:14 GMT FROM: Danny Steeghs at U.of Warwick/GOTO D.Steeghs, J.Lyman (U. Warwick), G.Ramsay (Armagh O.), M.Dyer (U. Sheffield), K.Ulaczyk, A.Levan, R.Cutter (U. Warwick) K. Ackley, D.Galloway, E.Rol (Monash U.), V.Dhillon (U. Sheffield), P.O'Brien, R.Starling (U. Leicester), S.Poshyachinda (NARIT), D.Pollacco (U. Warwick), E.Thrane (Monash U.) report on behalf of the GOTO collaboration: In response to the short-duration GRB 180728B (GCN 23056, 23057, 23060), the Gravitational-wave Optical Transient Observer (GOTO) observed the IPN triangulation region as reported in Svinkin et al. (GCN 23056). Observations were spread over four telescope array pointings, beginning 2018-07-29T21:24 UT (24.25 hours after the burst) and employ sets of 3x120s exposures in our wide L filter(400-700nm). These fields were repeated on 2018-07-30 to permit difference imaging analysis. Conditions were affected by dust, typically achieved a 5 sigma limiting magnitude of V=19.8-20.1 on 2018-07-29 and V=20.4-20.6 on 2018-07-30. We made use of the GLADE galaxy catalog to pay particular attention to possible source candidates near galaxies within 200 Mpc. We find no significant sources that could be credibly associated with the GRB. Furthermore, we see no evidence for a source at the position reported in Lipunov et al. (GCN 23065), noting that our observations started 4.7 hours later than the MASTER observations and thus the source may have faded below our detection limit by then. GOTO is operated at the La Palma observing facilities of the University of Warwick on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT) and the Instituto de Astrofisica de Canarias (IAC) https://goto-observatory.org/