GCN Circular 39972
Subject
LIGO/Virgo/KAGRA S250328ae: Swift XRT observations, 34 possible X-ray sources
Date
2025-03-31T14:04:07Z (3 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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 97 observations of the LVK error region for the GW trigger S250328ae
convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9), using the 'BAYESTAR' 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 3.0 ks to 38 ks after the LVK trigger,
and the XRT has covered 5.3 deg^2 on the sky (corrected for overlaps). This covers 68% of the
probability in the 'ligo-skymap-from-samples' skymap, and 72% after convolving with the 2MPZ galaxy
catalogue, as described by Evans et al. (2016, MNRAS, 462, 1591).
We have detected 34 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
* 34 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 | Detection Flag |
| S250328ae_X1 | 09h 37m 34.26s | +11d 00' 47.1" | 8.3" | POOR |
| S250328ae_X2 | 09h 37m 40.59s | +11d 15' 09.3" | 5.4" | REASONABLE |
| S250328ae_X3 | 09h 38m 01.16s | +11d 14' 02.6" | 6.5" | POOR |
| S250328ae_X5 | 09h 38m 05.54s | +11d 24' 32.5" | 6.5" | POOR |
| S250328ae_X6 | 09h 37m 57.28s | +11d 18' 05.4" | 6.5" | POOR |
| S250328ae_X8 | 09h 38m 05.52s | +11d 19' 00.3" | 9.1" | POOR |
| S250328ae_X9 | 09h 38m 34.55s | +11d 14' 28.5" | 6.4" | POOR |
| S250328ae_X10 | 09h 38m 37.34s | +11d 16' 12.5" | 9.9" | POOR |
| S250328ae_X11 | 09h 38m 13.89s | +11d 16' 52.4" | 9.5" | POOR |
| S250328ae_X12 | 09h 37m 35.17s | +10d 47' 58.5" | 6.6" | POOR |
| S250328ae_X13 | 09h 39m 29.53s | +11d 47' 43.2" | 6.0" | POOR |
| S250328ae_X14 | 09h 39m 57.31s | +12d 06' 58.4" | 8.7" | POOR |
| S250328ae_X15 | 09h 39m 31.81s | +12d 06' 34.6" | 7.9" | POOR |
| S250328ae_X16 | 09h 40m 10.28s | +11d 53' 17.1" | 6.7" | POOR |
| S250328ae_X17 | 09h 39m 48.52s | +11d 55' 48.6" | 7.4" | POOR |
| S250328ae_X18 | 09h 39m 45.99s | +11d 54' 42.0" | 7.5" | POOR |
| S250328ae_X19 | 09h 39m 39.79s | +11d 54' 32.3" | 6.6" | POOR |
| S250328ae_X20 | 09h 39m 23.21s | +11d 57' 42.7" | 6.6" | POOR |
| S250328ae_X21 | 09h 39m 41.64s | +11d 53' 24.9" | 13.1" | POOR |
| S250328ae_X22 | 09h 39m 45.64s | +11d 51' 32.9" | 9.1" | POOR |
| S250328ae_X23 | 09h 39m 28.69s | +11d 59' 35.2" | 8.2" | POOR |
| S250328ae_X24 | 09h 39m 51.34s | +11d 52' 24.6" | 6.2" | POOR |
| S250328ae_X25 | 09h 40m 08.46s | +11d 55' 52.5" | 8.9" | POOR |
| S250328ae_X26 | 09h 39m 51.33s | +11d 55' 00.9" | 6.6" | POOR |
| S250328ae_X27 | 09h 39m 34.55s | +12d 01' 50.9" | 7.2" | POOR |
| S250328ae_X28 | 09h 40m 17.23s | +11d 54' 47.3" | 9.7" | POOR |
| S250328ae_X29 | 09h 39m 31.19s | +11d 54' 35.1" | 8.4" | POOR |
| S250328ae_X30 | 09h 39m 39.91s | +12d 04' 29.3" | 15.4" | POOR |
| S250328ae_X31 | 09h 38m 40.15s | +11d 14' 00.3" | 5.7" | POOR |
| S250328ae_X32 | 09h 38m 17.68s | +11d 19' 25.6" | 13.5" | POOR |
| S250328ae_X33 | 09h 39m 18.34s | +11d 59' 40.6" | 8.9" | POOR |
| S250328ae_X34 | 09h 39m 52.80s | +11d 54' 16.5" | 8.0" | POOR |
| S250328ae_X35 | 09h 39m 34.91s | +12d 05' 56.6" | 9.0" | POOR |
| S250328ae_X36 | 09h 39m 29.20s | +12d 02' 51.1" | 10.7" | POOR |
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/S250328ae
This circular is an official product of the Swift XRT team.