TITLE: GCN CIRCULAR NUMBER: 41453 SUBJECT: LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz DATE: 25/08/20 17:43:21 GMT FROM: xjh@andrew.cmu.edu

Xander J. Hall (Carnegie Mellon U.), Robert Stein (UMD), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We requested ToO observations of AT 2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440).

The observation was taken with a midpoint time of 2025-08-19T11:48:55.755, approximately 30 hours after the trigger, with a duration of 3.5ks.

The source was not detected with XRT with an upper limit of 3.8 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.3 * 10^42 erg/s.

We clearly detect a source at the position of AT2025ulz in UVOT observations, conducted in the UVM2 filter, with a magnitude of m=22 AB. However, this flux is consistent with archival detections of the host galaxy in GALEX. We therefore find no evidence of transient UV emission in UVOT data.

We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.

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