TITLE:   GCN CIRCULAR
NUMBER:  42387
SUBJECT: EP/WXT 01709247295: GOTO optical afterglow candidate.
DATE:    25/10/23 08:44:08 GMT
FROM:    d.s.oneill@bham.ac.uk

M. E. Wortley, D. O'Neill, G. Ramsay, B. P. Gompertz, B. Godson, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, 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 Einstein Probe alert 01709247295.
Serendipitous imaging covered the position at 2025-10-23 05:25:22 (+2.9h post trigger) and targeted observations at 2025-10-23 05:36:01 UT, (+3.08h post trigger). The observations consisted of 4x45s and 4x90s exposures respectively 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.

A new optical source, GOTO25ixt, is identified within the WXT 90% localisation region with coordinates:

RA,DEC (J2000) =127.134217, 20.86431 | 08:28:32.21, +20:51:51.51

The source was initially detected with magnitude L = 18.57 ± 0.06 AB mag (+2.9h), with marginal fading to L = 18.64 ± 0.04 AB mag (+3.08h). The source was not detected in ATLAS forced photometry (Shingles et al 2021) at 2025-10-22 15:04:55 (11.44 hours pre-trigger), with a 5-sigma limiting magnitude of o > 19.45 AB mag.

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).