TITLE: GCN CIRCULAR NUMBER: 20964 SUBJECT: IceCube-170321A: Swift-XRT observations DATE: 17/03/31 19:53:20 GMT FROM: Azadeh Keivani at PSU A. Keivani (PSU), D. B. Fox (PSU), P.A. Evans (U. Leicester), J.A. Kennea (PSU), G. Tesic (PSU), D.F. Cowen (PSU), and J.P. Osborne (U. Leicester) report on behalf of the Swift-IceCube collaboration: Swift has observed the field of the IceCube EHE neutrino, IceCube-170321A (https://gcn.gsfc.nasa.gov/gcn3/20929.gcn3), utilizing the on-board 7-point tiling pattern to cover a region centered on the initial automated alert position RA,Dec (J2000) = (98.3268, -14.4861), with a radius of approximately 0.55 degrees. This covers about 21% of the 90% error region of the refined IceCube localization. Swift-XRT collected ~880 s per field of PC mode data per tile. The observations were taken between 14:09:02 on 2017-03-21 and 18:03:00 on 2017-03-21 (i.e. from 23.7 ks to 37.8 ks after the neutrino trigger), and covered 0.77 square degrees. Analysis using standard Swift tools yields >3-sigma detection of a single X-ray source, 1SXPS J063214.5-143300, which is also seen in previous observations of the field by Swift XRT, in a similar flux and spectral state. As such we conclude that there are no candidate X-ray counterparts to the possibly-cosmic high-energy neutrino within the covered region. Our 3-sigma upper limit on the count rate of any such counterpart is 3.6e-3 XRT ct s-1, which corresponds to a 0.3-10 keV flux of 1.48e-13 erg cm-2 s-1 for a typical afterglow/AGN power-law spectrum with photon index gamma=1.7 and equivalent hydrogen column density N_H=3e+20 cm-2. Details of the detected X-ray source are below. Source 1 ======== RA: 06h 32m 14.5s = 98.06042d Dec: -14d 32' 59.9" = -14.55000d Error: 6.0 arcsec (90% confidence radius) Count rate: (2.2 +/- 0.6) x 10^-2 ct s^-1 Flux: (9.0 +/- 2.5) x 10^-13 erg cm^-2 s^-1 Notes: This source has been previously detected by Swift (1SXPS J063214.5-143300) and the observed flux is consistent with the catalogued value.