B. Gompertz, K. Ackley, G. Ramsay, J. Lyman, D. O’Neill, A. Levan, K. Ulaczyk, M. Dyer, T. Killestein, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, D. Pollacco, K. Noysena, L. Nuttall, E. Pallé, report on behalf of the GOTO collaboration:
We report on observations of the fast fading transient candidate ZTF23aaoohpy/AT2023lcr (Swain et al, GCN 34022). The field was covered three times by the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2021) between 2023-06-17 and 2023-06-18. We detect the reported transient in an observation taken at 01:27:41 UT on 2023-06-18 (5.1 hours before the ZTF discovery) with a magnitude of 18.77+/- 0.06 in the GOTO L-band (400 – 700nm). The source was not detected in the previous epoch at 23:50:30 UT on 2023-06-17 to a 5-sigma limiting magnitude of L > 20.3. The measured rise is therefore > 1.5 magnitudes in 1h45m, and constrains the onset to between these two epochs. The GOTO L-band is approximately g+r, and therefore taking the first ZTF epoch of detection to be L ~ 19.5, our detection implies a fading rate of ~0.75 mag over 0.2 days - significantly faster than the 1mag/day between the two ZTF epochs.
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes 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), University of Turku, University of Portsmouth, University of Manchester and the Instituto de Astrofisica de Canarias (IAC)