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GCN Circular 33824

LIGO/Virgo S230518h: Swift XRT observations, 8 X-ray sources
2023-05-19T10:44:27Z (20 days ago)
P.A. Evans at U. Leicester <>
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 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 This circular is an official product of the Swift XRT team.
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