IceCube 135113_19489408.amon: Swift-XRT observations

Swift has observed the field of the IceCube Astrotrack Bronze 135113_19489408.amon. 4ks of cleaned XRT data were gathered between 04:55 and 07:03 UT on 2021 March 22. A point source is reliably detected at RA, Dec = (85.73812, -44.7522) which is equivalent to: RA(J2000.0) = 05h 42m 57.15s Dec(J2000.0) = -44d 45' 07.9" with an uncertainty of 5.1" (radius, 90% confidence). This position is consistent with the source 1RXS J054257.9-444452 in the RASS Faint Source Catalogue. Using a spectrum fitted to the XRT data, we find that the current 0.3-10 keV flux is 9.7 (+3.4, -2.7) ×10^-13 erg cm^-2 s^-1, which is about 3.7-sigma above the catalogued flux, suggesting variability or possibly an outburst: however, we caution that the spectral fit is very uncertain, and this flux increase should be viewed with caution. The source has no counterpart in SIMBAD, although it is a few arcseconds from a WISE object. Given the low statistical quality of the XRT spectral information, the lack of blazar classification for the RASS source, and the likelihood that the alert was caused by a muon, not an astrophysical neutrino (GCN Circ. 29688), we cannot claim this as a probable counterpart; however, should variability be detected at other wavelengths the case for this source as an interesting object would be strengthened.