TITLE: GCN CIRCULAR NUMBER: 39485 SUBJECT: LIGO/Virgo S250223dk: Swift XRT observations, 7 X-ray sources DATE: 25/02/26 08:33:05 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

P.A. Evans (U. Leicester), J.J. Delaunay (PSU), K.L. Page (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB), M. De Pasquale (University of Messina), S. Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia (ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N. Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha (NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien (U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:

Swift has carried out 1 observation of the combined LVK/Swift-BAT-GUANO error region for the GW trigger S250223dk, centred on the most-probable location (RA,Dec = 85.4260, -47.5420 [J2000]). We gathered 5.5 ks of data from 106 ks to 140 ks after the LVK trigger. This observations covers 73% of the probability in the combined skymap.

We have detected 7 X-ray sources. Each source is assigned a rank of 1-4 which describes how likely it is to be related to the GW trigger, with 1 being the most likely and 4 being the least likely. The ranks are described at https://www.swift.ac.uk/ranks.php.

We have found:

* 0 sources of rank 1 * 0 sources of rank 2 * 7 sources of rank 3 * 0 sources of rank 4

RANK 3 sources ==============

These are uncatalogued X-ray sources, however they are not brighter than previous upper limits, so do not stand out as likely counterparts to the GW trigger.

| Source ID | RA | Dec | Err90 | | S250223dk_X1 | 05h 41m 59.75s | -47d 21' 33.8" | 6.8" | | S250223dk_X2 | 05h 42m 11.19s | -47d 23' 57.3" | 6.4" | | S250223dk_X3 | 05h 41m 58.45s | -47d 33' 26.5" | 6.5" | | S250223dk_X4 | 05h 42m 19.59s | -47d 34' 02.3" | 6.9" | | S250223dk_X5 | 05h 42m 03.87s | -47d 42' 35.1" | 6.4" | | S250223dk_X6 | 05h 42m 01.25s | -47d 43' 06.8" | 6.3" | | S250223dk_X7 | 05h 41m 25.44s | -47d 26' 58.1" | 6.4" |

For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2, and photon index (Gamma)=1.7, unless otherwise stated.

The results of the XRT automated analysis, including details of the sources listed above, are online at https://www.swift.ac.uk/LVC/S250223dk

This circular is an official product of the Swift XRT team.

View this GCN Circular online at https://gcn.nasa.gov/circulars/39485. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...