TITLE: GCN CIRCULAR NUMBER: 41213 SUBJECT: LIGO/Virgo/KAGRA S250725j: GOTO pre-trigger detection of AT2025sib DATE: 25/07/31 14:27:23 GMT FROM: Nusrin Habeeb at University of Leicester nh312@leicester.ac.uk

B. Godson, N. Habeeb, K. Ackley, D. O’Neill, A. Kumar, G. Ramsay, 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 historical observations of counterpart candidate AT 2025sib (Hall et al. GCN 41177, McMahon et al. GCN 41206) with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024).

We find evidence of emission of the source starting at 2025-07-13 08:51 with a 3-sigma observation of L = 20.2 +/- 0.3 AB mag, with our final observation on 2025-07-21 10:11 with L = 20.8 +/- 0.3 AB mag at 3-sigma. We cannot rule out the variable nature of this source and find no evidence of the decay reported in Hall et al. GCN 41177 at these early times. We note that the source appears coincident with an object in the 2MASS Point Source Catalog, 2MASS15421160-3500115 (Cutri et al 2003).

GOTO last covered the field of S250725j serendipitously during survey mode on 2025-07-21. We did not perform follow-up for S250725j due to poor weather conditions at GOTO-South. In addition, we find no evidence of pre-detection emission for the other sources reported in GCN 41177, GCN 41206.

Continued analysis of additional candidates is ongoing.

Difference imaging was performed using deeper template observations. 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).

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