TITLE:   GCN CIRCULAR
NUMBER:  40542
SUBJECT: EP-WXT trigger 01709176712: GOTO observations confirm stellar counterpart variability
DATE:    25/05/23 19:51:55 GMT
FROM:    Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>

D. O'Neill, A. Kumar, K. Ulaczyk, B. P. Gompertz, G. Ramsay, S. Moran, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, 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 EP-WXT 01709176712 (Yang et al., GCN 40537). Targeted observations were performed beginning at 2025-05-23 12:56:35 UT (+0.2h post trigger) and continued through to 2025-05-23 15:17:24 UT (+2.55h post trigger). Each observation consisted of 4x90s 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.

We identify the stellar source reported by Yang et al., GCN 40537. Pre-trigger photometry from the GOTO archive shows the source exhibiting variability in the order of ±0.3 mag over the last 30 days. The latest pre-trigger photometry taken at 2025-05-22 21:44:20 UT (-15.0 h) has L = 15.38 ± 0.01 mag. Post trigger, the star rises to a peak L = 13.09 ± 0.01 mag at  2025-05-23 12:56:35 UT (+0.20 h) before fading to 14.29 ± 0.01 mag at 2025-05-23 15:17:24 UT (+2.55 h).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. Reported magnitudes are in the AB system.

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).