TITLE: GCN CIRCULAR NUMBER: 26475 SUBJECT: LIGO/Virgo S191216ap and IceCube neutrino: Swift XRT sources DATE: 19/12/17 15:03:57 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester), A. Keivani (Colombia U.), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S. Countryman (Colombia U.), I. Bartos (U. Florida), D. Fox (PSU), S. Marka (Colombia U.), Zsuzsa Marka (Colombia U.) S.D. Barthelmy (NASA/GSFC), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), P. Brown (TAMU), D.N. Burrows (PSU), S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), G. Cusumano (INAF-IASF PA), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia(ASDC), S.W.K. Emery (UCL-MSSL), P. Giommi (ASI), C. Gronwall (PSU), D. Hartmann (U. Clemson), H.A. Krimm (CRESST/GSFC/USRA), N.J. Klingler (PSU), 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. Leicester), C. Pagani (U. Leicester), K.L. Page (U.Leicester), M.J.Page (UCL-MSSL), D.M. Palmer (LANL), M. Perri (ASDC), J.L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift team: Swift has carried out 100 observations of the overlap between the LVC (GCN Circ. 26454) and IceCube (GCN Circ. 26463) error regions for the GW trigger S191216ap. The observations span from 22 ks to 42 ks after the LVC trigger, and the XRT has covered 10.2 deg^2 on the sky (corrected for overlaps). This covers 3.3% of the probability in the bayestar skymap, and 5.8% after convolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016, MNRAS, 462, 1591). It covers 65% of the probability contained within the combined GW and neutrino localisations. These pointings and associated metadata have been reported to the Treasure Map (Wyatt et al., GCN Circ. 26244; http://treasuremap.space/alerts?graceids=S191216ap). We have detected 3 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 * 2 sources of rank 4 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 are online at https://www.swift.ac.uk/LVC/S191216ap This circular is an official product of the Swift XRT team.