{
  "submittedHow": "email",
  "eventId": "LIGO/Virgo/KAGRA S250813k",
  "bibcode": "2025GCN.41384....1P",
  "createdOn": 1755242766104,
  "body": "K.L. Page (U. Leicester), P.A. Evans (U. Leicester), J.A. Kennea (PSU), A.\nTohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester),\nA.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL),\nS. Campana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina),\nS. Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia\n(ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N.\nKlingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha\n(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien\n(U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato\n(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),\nE. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:\n\nSwift has carried out 109 observations of the LVK error region for the GW trigger\nS250813k convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),\nusing the 'BAYESTAR' (version BAYESTAR.fits.gz) GW localisation map. As this is a 3D\nskymap, galaxy distances were taken into account in selecting which ones to observe.\nThe observations currently span from 2.0 ks to 164 ks after the LVK trigger, and the\nXRT has covered 7.9 deg^2 on the sky (corrected for overlaps). This covers 70% of\nthe probability in the 'ligo-skymap-from-samples' (version\nligo-skymap-from-samples.fits.gz) skymap, and 71% after convolving with the 2MPZ\ngalaxy catalogue, as described by Evans et al. (2016, MNRAS,  462, 1591).\n\nWe have detected 2 X-ray sources. Each source is assigned a rank of 1-4 which\ndescribes how likely it is to be related to the GW trigger, with 1 being the most\nlikely and 4 being the least likely. The ranks are described at\nhttps://www.swift.ac.uk/ranks.php.\n\nWe have found:\n\n  * 0 sources of rank 1\n  * 0 sources of rank 2\n  * 1 source of rank 3\n  * 1 source of rank 4\n\n\nRANK 3 sources\n==============\n\nThese are uncatalogued X-ray sources, however they are not brighter than previous\nupper limits, so do not stand out as likely counterparts to the GW trigger.\n\n| Source ID\t | RA\t\t  | Dec \t   | Err90   | Detection Flag\t|\n|  S250813k_X12  | 08h 54m 56.80s | +71d 23' 19.9\" |\t8.3\" |\t\t   GOOD |\n\n\nRANK 4 sources\n==============\n\nThese are catalogued X-ray sources, showing no signs of outburst compared to\nprevious observations, so they are not likely to be related to the GW trigger.\n\n| Source ID\t | RA\t\t  | Dec \t   | Err90   | Detection Flag\t|\n|  S250813k_X11  | 09h 17m 14.09s | +75d 30' 53.0\" |\t7.1\" |\t\t   GOOD |\n\nFor all flux conversions and comparisons with catalogues and upper limits from other\nmissions, we assumed a power-law spectrum with NH=3x10^20 cm^-2, and photon index\n(Gamma)=1.7, unless otherwise stated.\n\nThe results of the XRT automated analysis, including details of the sources listed\nabove, are online at https://www.swift.ac.uk/LVC/S250813k\n\nThis circular is an official product of the Swift XRT team.\n\n\n",
  "circularId": 41384,
  "subject": "LIGO/Virgo/KAGRA S250813k: Swift XRT observations, 2 X-ray sources",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}