TITLE: GCN CIRCULAR NUMBER: 41201 SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources DATE: 25/07/30 10:45:29 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB), J.J. Delaunay (PSU), 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 171 observations of the LVK error region for the GW trigger S250727dc convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9), using the 'bayestar.multiorder.fits,1' GW localisation map. As this is a 3D skymap, galaxy distances were taken into account in selecting which ones to observe. The observations currently span from 3.9 ks to 157 ks after the LVK trigger, and the XRT has covered 11.5 deg^2 on the sky (corrected for overlaps). This covers 70% of the probability in the imrpoved 'Bilby.offline0.multiorder.fits' skymap, and 69% after convolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016, MNRAS, 462, 1591).

In total, we have detected 8 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 (see note) * 3 sources of rank 3 * 5 sources of rank 4

NOTE: The online system reports a rank 2 source: Source S250727dc_X1. However, this in the known blazar BL Lac (the prototype of its class of object). During our observation, its flux was significantly increased compared to the mean value in the LSXPS catalogue, hence the automated classification. However, it is known to be highly variable (e.g. https://www.swift.psu.edu/monitoring/source.php?source=BLLacertae) and the intensities in LSXPS are drawn solely from PC-mode data so are biased towards the lower-intensity states. We have thus reclassified this as rank 4 for this circular.

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 | Detection Flag | | S250727dc_X5 | 06h 23m 38.04s | -35d 38' 24.1" | 7.7" | GOOD | | S250727dc_X6 | 06h 28m 07.71s | -37d 10' 10.3" | 9.5" | REASONABLE | | S250727dc_X7 | 06h 18m 45.03s | -35d 18' 15.1" | 7.0" | GOOD |

RANK 4 sources ==============

These are catalogued X-ray sources, showing no signs of outburst compared to previous observations, so they are not likely to be related to the GW trigger.

| Source ID | RA | Dec | Err90 | Detection Flag | | S250727dc_X1 | 06h 27m 06.55s | -35d 29' 15.0" | 3.8" | GOOD | | S250727dc_X3 | 06h 34m 35.33s | -37d 17' 12.1" | 8.3" | GOOD | | S250727dc_X8 | 06h 36m 18.37s | -37d 22' 33.4" | 4.5" | GOOD | | S250727dc_X9 | 06h 19m 53.43s | -35d 24' 28.2" | 7.7" | GOOD | | S250727dc_X10 | 06h 20m 08.64s | -34d 37' 54.4" | 5.8" | GOOD |

In addition, we took a 1.9 ks exposure of the UVOT source reported by Kuin et al. (GCN Circ. 41187). No XRT counterpart was found, with a 3-sigma upper limit of 3.9e-3 ct/sec (approx 1.6e-13 erg cm^-s s^-1, 0.3-10 keV).

The results of the XRT automated analysis, including details of the sources listed above, are online at https://www.swift.ac.uk/LVC/S250727dc

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

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