GCN Circular 42681

Subject
LIGO/Virgo/KAGRA S251112cm: Swift XRT observations, 20 X-ray sources
Event
LIGO/Virgo/KAGRA S251112cm
Date
2025-11-14T15:15:33Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
K.L. Page (U. Leicester), P.A. Evans (U. Leicester), J.A. Kennea (PSU), A.
Tohuvavohu (Cosmic Frontier Labs), 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 105 observations of the LVK error region for the GW trigger
S251112cm 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 7.6 ks to 117 ks after the LVK trigger, and the
XRT has covered 12.7 deg^2 on the sky (corrected for overlaps). This covers 1.3% of
the probability in the 'BAYESTAR' (version BAYESTAR.fits.gz) skymap, and 20% after
convolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016,
MNRAS,	462, 1591).

We have detected 20 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
  * 13 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   | Detection Flag	|
|  S251112cm_X1  |    11 09 55.90 |    +36 08 20.7 |	8.0" |		   POOR |
| S251112cm_X38  |    23 47 03.18 |    -28 07 08.2 |	9.5" |		   POOR |
| S251112cm_X53  |    11 08 18.65 |    +36 11 04.6 |	8.1" |		   POOR |
| S251112cm_X72  |    12 22 09.27 |    +55 52 20.4 |	5.9" |		   GOOD |
| S251112cm_X73  |    01 08 29.26 |    -46 59 48.1 |	8.9" |		   POOR |
| S251112cm_X78  |    23 48 35.23 |    -28 14 05.5 |	9.8" |		   POOR |
| S251112cm_X79  |    23 48 06.04 |    -28 04 35.1 |   12.3" |		   POOR |


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	|
|  S251112cm_X2  |    11 12 06.93 |    +35 27 45.6 |	6.8" |		   GOOD |
|  S251112cm_X3  |    09 20 46.26 |    -08 03 21.4 |	5.5" |		   GOOD |
|  S251112cm_X5  |    23 51 37.99 |    -28 18 18.1 |	7.6" |		   POOR |
| S251112cm_X50  |    11 37 24.49 |    +47 27 44.7 |	7.4" |		   GOOD |
| S251112cm_X51  |    01 08 20.79 |    -47 07 36.3 |	7.4" |		   POOR |
| S251112cm_X52  |    01 07 42.60 |    -46 54 24.5 |	6.1" |		   GOOD |
| S251112cm_X67  |    11 11 04.66 |    +35 13 59.3 |	6.9" |		   GOOD |
| S251112cm_X68  |    22 44 47.97 |    -00 06 17.5 |	8.8" |		   GOOD |
| S251112cm_X69  |    02 36 19.70 |    -54 51 50.8 |	5.8" |		   GOOD |
| S251112cm_X70  |    13 06 57.05 |    +59 58 48.1 |   10.7" |	     REASONABLE |
| S251112cm_X71  |    12 23 00.00 |    +55 40 04.4 |	5.1" |		   GOOD |
| S251112cm_X87  |    11 12 01.25 |    +37 58 26.3 |	6.1" |		   GOOD |
| S251112cm_X88  |    11 04 27.52 |    +38 12 36.6 |	3.5" |		   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/S251112cm

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