A. Keivani (Columbia U.), P. A. Evans (U. Leicester), D. B. Fox (PSU), J.
A. Kennea (PSU), D. F. Cowen (PSU), J. P. Osborne (U. Leicester), F. E.
Marshall (GSFC), Marcos Santander (U. Alabama), Miguel Mostafa (PSU), and
Hugo Ayala (PSU) report:
The Neil Gehrels Swift Observatory observed the field of the IceCube HESE
astrophysical neutrino candidate IceCube-190221A (revision 1,
https://gcn.gsfc.nasa.gov/gcn3/23918.gcn3) beginning Feb 21.6 UT (6.55h
after the neutrino detection), utilizing its onboard 19-point tiling
pattern to cover a region centered on R.A., Dec. (J2000) = 268.81d,
-17.04d, with a radius of approximately 0.8 degrees; estimated
90%-containment radii for this event are 0.5 deg to 1.8 deg depending on
position angle. Earlier observations targeting the pre-revision
localization R.A., Dec. (J2000) = 267.3650d, -16.9379d beginning ~2 hours
after the neutrino arrival time have also been executed, and are not
Swift-XRT collected ~400 s per field of PC mode data per tile between 14:58:40
UT and 21:34:12 UT on 21 February (6.55h to 13.14h after the neutrino
detection) over 2.1 square degrees. Data have been reduced using the
analysis approach and software routines of Evans et al. 2014 (ApJS, 210,
Three X-ray sources are detected in these data, two of which likely
correspond to the stars HD 162561 (2MASS J17521921-1658102) and TYC
6254-1118-1 (2MASS J17531121-1725423). The third X-ray source is located at
R.A. 17h 53m 50.24s, Dec. -16d 18��� 45.1��� (J2000) with an uncertainty of
4.8��� (90%-confidence radius) and matches the known X-ray source XMMSL2
J175349.9-161846 from the XMM-Newton slew survey (3.7��� distance). The
source���s mean observed XRT count rate is 3.9 (+1.5, -1.2) e-2 ct s^-1,
which is 1.7-sigma below the XMMSL2 catalog flux.
The 3-sigma upper limit on the count rate of any point-like counterpart
over the rest of the tiled region is 0.02 ct s^-1, which corresponds to a
0.3-10 keV flux of 8.0e-13 erg cm^-2 s^-1 for a typical AGN spectrum
(nH=3e20 cm^-2, Gamma=1.7).