TITLE: GCN CIRCULAR NUMBER: 41474 SUBJECT: LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits DATE: 25/08/21 19:06:59 GMT FROM: Sylvio Klose at TLS Tautenburg klose@tls-tautenburg.de
S. Klose, B. Stecklum, A. Nicuesa Guelbenzu, and F. Ludwig (all Tautenburg) report:
We observed the optical transient ZTF25abjmnps/AT2025ulz (Stein et al., GCN 41414), possibly associated with the gravitational wave event S250818k (GCN 41437), with the Tautenburg 1.34-m Schmidt telescope using the Sloan filter set.
Observations were performed on 20-08-2025 between 20:06 and 20:39 UT as well as between 21:57 and 22:30 UT. They reached a 3-sigma upper limit in r and i of 22.3 and 22.4 mag (AB), respectively. Photometric calibration was performed using Pan-STARRS DR2.
At the position of the optical transient (OT) discovered by Stein et al. (GCN 41414; see also Busmann et al., GCN 41421; Hall et al., GCN 41433), no point source superimposed the underlying host galaxy was detected / could be identified.
Given the closeness of the OT to the host galaxy's nucleus (Liu et al., GCN 41461) we caution that the photometric limits are preliminary.
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