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.