TITLE: GCN CIRCULAR NUMBER: 40643 SUBJECT: EP/WXT 01709177873: GOTO observations confirm stellar variability DATE: 25/06/06 10:10:51 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com> A. Kumar, D. O'Neill, G. Ramsay, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the 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 EP/WXT alert 01709177873. Targeted observations were performed beginning at 2025-06-05 21:27:07 UT (+2.59 hr post trigger) and continued through to 2025-06-05 23:43:10 UT (+4.86 hr post trigger). Each observation consisted of 4x90 s exposures in the GOTO L-band (400-700 nm). 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. No new transients associated with the alert were identified, except for one source within the localisation region: RX J1553.0+4457, a low mass star + white binary at RA = 15:53:04.8, Dec = +44:57:44.6. This source exhibited significant variability, brightening by ~0.6 mag relative to the last pre-trigger image taken on 2025-05-30 at 00:18:22 UT (6.77 days pre-trigger), reaching 13.70 ± 0.01 mag in the first post-trigger image at 2025-06-05 21:27:07 UT (+2.59 hr). It then faded to 14.10  ±  0.01 mag by the final observation at 2025-06-05 23:43:10 UT (+4.86 hr post-trigger). All the reported magnitudes are in the GOTO L-band and the AB system. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. 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 and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/40643. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...