TITLE:   GCN CIRCULAR
NUMBER:  41535
SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Observations Show Continued Reddening of AT 2025ulz
DATE:    25/08/25 09:09:19 GMT
FROM:    Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>

Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), and Mansi Kasliwal (Caltech) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414)  possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, i, and J bands simultaneously starting at 2025-08-24T19:33:25 for 20 x 180 s with a few interruptions due to clouds. The difference imaging with templates from the Legacy Survey for the g-band and PS1 for the i-band, we report that AT 2025ulz has g - i ~ 1.7 AB mag.

We note our differenced i-band brightness is consistent with observations reported by Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518).

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

Further analysis is underway.

We thank Christoph Ries from the Wendelstein Observatory staff for obtaining these observations.