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GCN Circular 33824

Subject
LIGO/Virgo S230518h: Swift XRT observations, 8 X-ray sources
Date
2023-05-19T10:44:27Z (a year ago)
From
P.A. Evans at U. Leicester <pae9@leicester.ac.uk>
P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL),
S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), P. D'Avanzo (INAF-OAB), R.A.J.
Eyles-Ferris (U. Leicester), C. Gronwall (PSU), D. Hartmann (U. Clemson), N.J.
Klingler (NASA/GSFC), N.P.M. Kuin (UCL-MSSL), A.Y. Lien (GSFC/UMBC), F.E.
Marshall (NASA/GSFC), A. Melandri (INAF-OAB), J.A. Nousek (PSU), S.R. Oates (U.
Birmingham), P.T. O'Brien (U. Leicester), J.P.Osborne (U. Leicster), K.L. Page
(U.Leicester), M.J.Page (UCL-MSSL), 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 60 observations of the LVC error region for the GW trigger
S230518h 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 7.8 ks to 35 ks after the LVC trigger, and
the XRT has covered 6.0 deg^2 on the sky (corrected for overlaps). This covers
1% of the probability in the 'ligo-skymap-from-samples' skymap, and 4.7% after
convolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016,
MNRAS, 462, 1591). These pointings and associated metadata have been reported to
the Treasure Map.

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
  * 1 source of rank 2
  * 1 source of rank 3
  * 6 sources of rank 4
  
The 'rank 2' source was identified as such because it is uncatalogued in X-rays, and in
the initial analysis its flux was found to be significantly above the upper-limit from
the ROSAT All Sky Survey. However, this assumes a standard spectral model; using a model
fitted to the data from source 2 for this conversion we find that the XRT observed flux
is consistent with the ROSAT non-detection and so cannot be identified as a transient.


Details of all of the detected sources, by rank, are given below. All
coordinates are in J2000.

RANK 2 sources
==============

These are uncatalogued X-ray sources, which in the inital X-ray analysis had fluxes above the
historical 3-sigma upper limits.

| Source ID      | RA             | Dec            | Err90   |
|   S230518h_X8  | 22h 19m 10.78s | -84d 49' 08.0" |    7.8" |



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   |
|   S230518h_X6  | 07h 46m 12.93s | -54d 02' 40.3" |    7.8" |


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   |
|   S230518h_X1  | 20h 40m 07.33s | -71d 15' 02.7" |    4.6" |
|   S230518h_X2  | 20h 41m 59.36s | -73d 19' 09.5" |    7.7" |
|   S230518h_X3  | 20h 02m 16.15s | -83d 26' 41.9" |    6.3" |
|   S230518h_X4  | 07h 45m 07.88s | -54d 04' 50.7" |   17.1" |
|   S230518h_X5  | 07h 45m 09.54s | -54d 05' 45.3" |   15.4" |
|   S230518h_X7  | 07h 02m 12.02s | -86d 43' 48.3" |    6.7" |

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

The results of the XRT automated analysis, including details of the sources
listed above, are online at https://www.swift.ac.uk/LVC/S230518h

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



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