GCN Circular 41641
Subject
EP250827b: Swift observations of AT 2025wkm
Event
Date
2025-09-01T20:57:31Z (11 days ago)
From
xjh@andrew.cmu.edu
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Tomas Ahumada (Caltech), and Robert Stein (UMD) on behalf of the ZTF collaboration:
We requested Swift ToO observations of the X-ray flare EP250827b and optical transient AT 2025wkm reported by Schroeder et al. (GCN 41635).
The observation was taken with a midpoint time of 2025-08-31 at 21:13:29 UT, approximately 4.62 days after the flare, with a duration of 4ks.
The source was not detected with XRT with an upper limit of 2.5 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift reported by Sevilla & Ho (GCN 41639) of z = 0.1200 this gives a rough luminosity upper limit of 4.5 * 10^42 erg/s.
We clearly detect a source at the position of AT 2025wkm in UVOT observations. We use a synthetic galaxy model to produce subtracted magnitudes and correct for galactic extinction. We report in the UVW1 filter, a magnitude of m= 21.7 +/- 0.4 AB and in the UVM2 filter, a magnitude of m = 22.2 +/- 0.3.
We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.