LIGO/Virgo/KAGRA S250813k

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.

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250813k (GCN Circular 41336
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 ). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250813k

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 13 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h02m, +74d05m, 3.72d, 1.09d, 106.41d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 708 +/- 159 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc
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  and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
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The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250813k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-13 02:12:00.444 UTC (GPS time: 1439086338.444). The candidate was found by the cWB [1], GstLAL [2], MBTA [3], MLy [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250813k is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250813k

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 21 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h01m, +73d31m, 5.03d, 1.32d, 107.20d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 932 +/- 209 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
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 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004
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  and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
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 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
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 [4] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
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 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
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 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
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 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
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 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.0240
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