TITLE: GCN CIRCULAR NUMBER: 24405 SUBJECT: IceCube-190504A: Swift-XRT Follow-up Observations DATE: 19/05/06 11:00:22 GMT FROM: Azadeh Keivani at Columbia U IceCube-190504A: Swift-XRT Follow-up Observations A. Keivani (Columbia U.), D. B. Fox (PSU), P. A. Evans (U. Leicester), J. A. Kennea (PSU), M. Santander (U. Alabama), & F. Krauss (GRAPPA/API, University of Amsterdam) report: The Neil Gehrels Swift Observatory observed the field of the IceCube HESE astrophysical neutrino candidate event IceCube-190504A (revision 1, https://gcn.gsfc.nasa.gov/gcn3/24392.gcn3) beginning May 4, 20:09:37 UT (1.74 hours after the neutrino arrival time). Swift utilized its onboard 19-point tiling pattern to cover a region centered on R.A., Dec. (J2000) = 65.7866d, -37.4431d, with a radius of approximately 0.8 deg; estimated 90%-containment radius for this event is 1.23 deg. Swift-XRT collected ~800 s per field of PC mode data per tile between 20:12:40 UT on 4 May and 09:08:01 UT on 5 May. Data have been reduced using the analysis approach and software routines of Evans et al. 2014 (ApJS, 210, 8). Several X-ray sources are detected in these observations, most of which are previously uncataloged and relatively low in flux, consistent with expectations for serendipitous (unrelated) sources over a region of this extent. The highest-significance X-ray source is located at R.A. 66.17568d, Dec. -37.93938d (J2000), 35 arcmin away from the neutrino direction, and matches the known (SIMBAD) X-ray source QSO B0422-380 (1.4 arcsec away) as well as 1SXPS J042442.5-375618 (6 arcsec away). We recover it at a mean observed XRT count rate of 3.2 (+0.8, -0.7) e-2 ct s^-1, consistent with (1.5-sigma above) the cataloged flux from 1SXPS. Although the brightness of this object does not look significantly elevated over the previous Swift observations, it is catalogued as a blazar and would be interesting if there is other electromagnetic emission from this source. Fermi-LAT has reported (GCN Circular #24401) significant emission at gamma ray over a timescale of one week before the neutrino arrival time (at a flux level consistent with the 10 years average) but no recent remarkable activity at gamma rays. Excluding identified sources, the 3-sigma upper limit on the count rate of any point-like counterpart over the rest of the tiled region is 0.01 ct s^-1, which corresponds to a 0.3-10 keV flux of 4.0e-13 erg cm^-2 s^-1 for a typical AGN spectrum (nH=3e20 cm^-2, Gamma=1.7). Coordinates, count rates, and possible counterparts for the X-ray sources are provided below. Source, RA, Dec, Error_90 (arcsec), XRT count rate (ct s^-1), Possible counterparts ------------------------------------------------------------------------------------------------------------------ #1: 66.17568d, -37.93938d, 7.9, 3.2 (+0.8, -0.7) e-2, QSO B0422-380 (1.4 arcsec away) & 1SXPS J042442.5-375618 (6 arcsec away) ------------------------------------------------------------------------------------------------------------------ #2: 66.39382d, -37.70566d, 6.6, 1.3 (+0.7, -0.5) e-2, 1RXS J042532.5-374212 (25.4 arcsec away) ------------------------------------------------------------------------------------------------------------------ #3: 65.19589d, -37.26399d, 9.5, 1.0 (+0.6, -0.4) e-2, 1RXS J042449.3-365537 (13.2 arcsec away) ------------------------------------------------------------------------------------------------------------------ #4: 66.20972d, -36.92833d, 6.3, 1.4 (+0.6, -0.5) e-2, -- ------------------------------------------------------------------------------------------------------------------ #5: 65.70905d, -37.94807d, 7.5, 1.2 (+0.6, -0.5) e-2, -- ------------------------------------------------------------------------------------------------------------------ #6: 66.39458d, -37.70534d, 7.9, 1.3 (+0.7, -0.5) e-2, 1RXS J042532.5-374212 (27.1 arcsec away) ------------------------------------------------------------------------------------------------------------------ #7: 65.29548d, -38.09169d, 10.5, 1.3 (+0.6, -0.5) e-2, 1RXS J042110.9-380533 (2.9 arcsec away)