GCN Circular 41384

K.L. Page (U. Leicester), 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),
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 109 observations of the LVK error region for the GW trigger
S250813k convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),
using the 'BAYESTAR' (version BAYESTAR.fits.gz) 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 2.0 ks to 164 ks after the LVK trigger, and the
XRT has covered 7.9 deg^2 on the sky (corrected for overlaps). This covers 70% of
the probability in the 'ligo-skymap-from-samples' (version
ligo-skymap-from-samples.fits.gz) skymap, and 71% after convolving with the 2MPZ
galaxy catalogue, as described by Evans et al. (2016, MNRAS,  462, 1591).

We have detected 2 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
  * 1 source of rank 3
  * 1 source 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   | Detection Flag	|
|  S250813k_X12  | 08h 54m 56.80s | +71d 23' 19.9" |	8.3" |		   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	|
|  S250813k_X11  | 09h 17m 14.09s | +75d 30' 53.0" |	7.1" |		   GOOD |

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/S250813k

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