TITLE: GCN CIRCULAR NUMBER: 41436 SUBJECT: LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz) DATE: 25/08/20 05:23:52 GMT FROM: Mansi Kasliwal at Caltech mansikasliwal@gmail.com

Viraj Karambelkar (Caltech), Mansi M. Kasliwal (Caltech) and Xander J. Hall (CMU) report on behalf of the larger ZTF and GROWTH collaborations

We observed ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) starting UTC 2025-08-19 06:42 with the LRIS spectrograph on Keck I. We derive a redshift of 0.0848 based on host galaxy emission lines - this is consistent with the 3D GW localization for S250818k. Subtracting host galaxy light, we see a blue continuum excess from the transient, and no tell-tale supernova-like features. The data in-hand thus far appears consistent with a kilonova. However, continued monitoring (such as Busmann et al. GCN 41421, Hall et al. GCN 41433) is encouraged to rule out any future re-brightening from a faint and fast supernova Type IIb (e.g., Barna et al. 2025, arXiv:2506.15900). Panchromatic follow-up is ongoing and encouraged.

View this GCN Circular online at https://gcn.nasa.gov/circulars/41436. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...