TITLE: GCN CIRCULAR NUMBER: 41461 SUBJECT: LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz DATE: 25/08/21 09:00:21 GMT FROM: Zhengyan Liu at USTC <ustclzy@mail.ustc.edu.cn> Z. Y. Liu, Z. L. Xu, D. Z. Meng, J.-A. Jiang, W. Zhao, M. X. Cai, Z. G. Dai, L. L. Fan, N. Jiang, X. Kong, T. G. Wang (USTC), Y. Z. Fan, J. J. Geng, Z. P. Jin, X. F. Wu (PMO) report on behalf of the WFST Collaboration: We report the pre-discovery upper limits and follow-up photometry results of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave (GW) event S250818k (GCN 41437), using the Wide Field Survey Telescope (WFST Collaboration; Wang et al., 2023). After checking WFST archive data, we found u- and g-band 30s exposures taken at MJD 60901.66, about 3.4 days prior to the GW event trigger. The images were processed with the WFST pipeline, using stacked WFST multi-exposures taken in February 2025 as templates. We do not found any residuals in difference images in the two bands. We report the following pre-discovery 3-sigma upper limits and time since the S20250818k trigger (t-t0): MJD t-t0 (days) AB mag Band 60901.64 -3.42 > 22.1 u 60901.67 -3.38 > 22.4 g We conducted WFST follow-up observations of AT2025ulz with 120s exposures in g and r bands and a 180s exposure in u band beginning at 2025-08-20T14:24:03 UTC. After image subtraction, a clear source was detected in g band, and faint sources detected in u and r bands, which are close to the detection limit. The preliminary AB magnitudes of AT2025ulz are (all photometries are not corrected for Galactic extinction): MJD t-t0 (days) AB mag Band 60907.60 2.54 22.71+/-0.10 g 60907.61 2.55 22.78+/-0.16 r 60907.65 2.59 23.24+/-0.24 u The g-r and u-g color from WFST observations are approximately -0.1 mag and 0.5 mag, respectively, which became bluer compared with the previous reports (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454). Additionally, our results show a re-brightening trend for AT2025ulz, which may be related to the shock-cooling process mentioned in Gillanders et al. (GCN 41454). We also note that AT2025ulz is significantly more luminous than AT 2017gfo at the same phase, especially in g and u bands assuming a host redshift 0.0848 reported by Karambelkar et al. (GCN 41436). The angular separations between the detected source in g band and the host center are approximately 0.85 and 0.83 arcseconds using host coordinates in PS1 DR2 (Chambers et al., 2016) and SDSS DR16 (Ahumada et al., 2020), respectively. Given the close distance between the source and the bright host nucleus, and the peculiar photometric evolution in the past 24 hours, we suggest that the variability might be influenced by the host-nuclear activity. Further deep multiband imaging follow-ups are highly encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41461. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...