TITLE: GCN CIRCULAR NUMBER: 41187 SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm DATE: 25/07/28 15:31:25 GMT FROM: Paul Kuin at MSSL npkuin@gmail.com

N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C. Gronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall (NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of Messina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì (INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J. Delaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR), R.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri (PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha (NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DTU Space), P. O’Brien (U. Leicester), J.P. Osborne (U. Leicester), T. Partosan (NASA/GSFC), M. Perri (ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB), E. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:

Swift observed the high-probability region of LIGO/Virgo/KAGRA S250725dc starting 65.3 minutes after the trigger date. A new UVOT source was found in the U band with U = 19.9 +/- 0.3 mag (ABmag) image taken on 2025-07-28 00:34_55 UT at the following position:

RA = 97.47619, Dec = -37.23307 deg, or RA = 06:29:54.3, Dec = -37:13:55.45 in sexagesimal notation.

We do not find any source on that location in DSS, using the Vizier photometry tool, or as a nearby object known to the Minor Planet Center MPChecker tool. The object has been registered as 2025smm by the Transient Name Server.

A further Swift observation has been planned.

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