TITLE:   GCN CIRCULAR
NUMBER:  28940
SUBJECT: IceCube-201120A: Swift-XRT non-detection of 4FGL J2028.6+4110e
DATE:    20/11/21 10:04:32 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

D. B. Fox (PSU), A. Keivani (Columbia U.), J.P. Osborne (U. Leicester), F. Krauss (PSU), T. Gregoire (PSU), P.A. Evans (U. Leicester), J.A. Kennea (PSU) and H. A. Ayala Solares (PSU),  D.F. Cowen (PSU) and J. DeLaunay (PSU) report:

Swift observed the location of 4FGL J2028.6+4110e, the Fermi-LAT source closest to the location of IceCube 201120A (IceCube Collaboration; GCN Circ. 28927) for 3.5 ks between 17:10UT and 20:05UT on 2020 November 20. No sources are found in the XRT field of view. 

The background level varies significantly across the field of view, due to the presence of stray light arising from the out-of-field bright source 2SXPS J203225.7+405727 (=1RXH J203225.7+405727) which is approximately 45’ away from the XRT boresight. This stray light was modelled automatically when searching for sources, using the new online source detection tool within https://www.swift.ac.uk/user_objects.

Taking this variation into account the 3-sigma upper limit in the XRT field of view varies between 3.7 x 10^-3 ct/sec and 4.5x10^-3 ct/sec. Assuming a power-law spectral photon index of 1.7, and Galactic absorption (1.46 x 10^22 cm^-2; Willingale et al, 2013), this corresponds to an observed 0.3-10 keV flux of 2.7 x 10^-13 erg cm^-2 s^-1 to 3.3 x 10^-13 erg cm^-2 s^-1